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Better analogy for newer is better fixation would be that, because rubber tires are century old idea, car with steel wheels and some marketing BS hype must be better. Also because steering wheel is equally old, car with "modern digital" controls of one button turning steering wheels at fixed speed toward right and other button toward left would surely be better, right? Important breakthroughs in headphones were made long ago. Open design which brought kinda of revolution to sound quality is actually 50 years old. And newer isn't automatically better. For example mechanical design of current HD500-models with that fashionable looking single sided ear cup attachment is worser than old. Also shiny fashion plastics tend to be more brittle than old standard black plastic. Old desing HD590 (my first full size headphone) survived decade of use without mechanical wear except in skinny cable. While designed for fashion HD595 started breaking in before three years, as consequence of flawed mechanical design and brittle bling bling plastic. Unless you want to import from US just use Euros, like most EU countries from which you're free to buy without any import customs worries. Ang gaming is actually the most demanding use if you want best immersion capabilities. Stereo music simply forgives lots of deficiencies compared to binaural sound.

That motherboard should have better design integrated sound card more resilient against interference. And for headphone gaming its bundled software is actually very good offering good binaural sound simulation. That USB dongle is definitely of lot lower quality, though for such cheap speakers it's no doubt adequate. So with some software it would be possible to use it for speakers and integrated four headphones.

For multithreaded work that CPU would be upgrade. But for gaming that almost decade old Nehalem architecture is simply outdated and nothing is going to bring it more up to date.

Quality difference depends on case and also likely on how wiring for front panel header is done on motherboard. But switching between them instead of having sound come out from both might be harder in Windows. Though there are physical audio switches, which can be used to connect two devices to one output connecting only one at time to output. Also there are sound cards with separate headphone otu and line out and ability to switch signal between them in software.

Of course its possible that case has faulty headphone output connector, but headset having combined TRRS plug might be well behind that connection problem. So you should try adapter like this https://www.amazon.com/Headphones-Convertors-Headphone-Microphone-Simultaneously/dp/B01M8G1U0C

Beyerdynamic uses steel headband inside cushion which doesn't mind bending it to more comfortable clamping force. And weight is certainly one aspect for comfort. AKGs would be actually light, though "automatic" head band adjustment might not work so well, unless having fitting head shape/size. Also they're mostly open. So do you have noisy or quiet environment? I guess use is mostly gaming.

Those are meant to connect to motherboard's pin header for power and reser button of case. But unless you connect them in parallel buttons of case won't work. Not sure if there are any ready splitters for connecting wires of two "buttons" to one header. This kind PCB breadboard/prototype board wires with female connector in one and and male in other could be used to make splitter cables: https://www.digikey.com/product-detail/en/sparkfun-electronics/PRT-12794/1568-1511-ND/5993859

Connecting RCA plugs to other channel's input is exactly the best way to get correct left and right direction in this situation. Speaker's amplifier circuitry simply doesn't know the difference and wouldn't care. And it doesn't need anything in PC. Trying to swap channels in software without also doing same for headset might be hard. While that K.GUSS advertises lots of hype components actual performance isn't likely anything that level. So that C-Media sound chip based USB dongle might give pretty much same sound. Though with that price it's certain to be very basic chip.

Do you have space problem or why such tiny screen? It's simply completely unbalanced to graphics card. Even if size wouldn't be cramped that medieval PR BS vertical resolution would make any use masochistic... With need to keep constantly scrolling up and down when reading web pages. 24" 1920x1200 would be absolute minimum I would consider minimum tolerable for any use. (and even that would be bad after nearly five years at 30" 2560x1600) Compared to your current monitor physical size difference would be this: http://www.displaywars.com/21-inch-16x9-vs-24-inch-16x10 Also that other aspect of gaming immersion, is something I wouldn't trust to be worth of even 10$ in sound quality. Gaming brands sell cheap bling blinged trinket production with huge profit margins. Obscene profit margins are reason for all these gamnig brands appearing in ten years. While you can get good headsphones from actual audio maker with only small part of price of technically old in couple years graphics card. (last couple yars have been exception with lack of competition in GPUs) Though if you play only some strategy games only thing needed from sounds in those is having sound... Unlike in fps games where being able to tell direction and even sense distance is useful for immersion.

Do you have quiet or noisy environment? And do you want accurate neutral sound, or stronger bass? If in quiet environment open ear cup design would also improve comfort by insulating less heat. There would be some very good neutral bass open headphones for that price. Audio-Technica ATH-AD700x would be very good for distinguishing details like foot steps etc, though not so great for bass immersion.

Cable is likely the weakest point if mishandled. So like said above try to avoid tight bending stresses on it and especially near connectors. Also depending on cable length try to make sure that cable isn't falling to floor as big loop. Would be pretty easy to get that loop caught on something/step on it. Of course use both hands to take them off from head and set them on table gently etc, instead of just pulling with one hand and dropping them to table.

Do you want maximal details at expense of "fun factor"/bass immersion? Or some compromising in details for good bass impact? Also do you actually need microphone, or say headset because gaming stuff is called that? I got that cured by fashionable looking HD595 starting to drop pieces of plastics after 2½ years... Giving look inside poor mechanical design made fashion first instead of durability. And you should keep something cheap expendable stuff nearby just for throwing that around instead of more valuable things.

5.1 or 7.1 makes no real difference to amount of available spatial information: Sound coming directly from left or right simply has identical signal in front and rear channels of that side in 5.1. Asus isn't exactly great for updating drivers of their sound cards. They even ditched Phoebus line in under five years. Relying on sound chips and likely drivers made by C-Media might have something to do it. (latest cards actually use USB sound chip through PCI-e bridge chip) Creative has also surpassed TPA6120 based headphone amps of Asus in AE-5. ZxR just isn't worth its price unless needing all that connectivity, with its TPA6120 based headphone amp being inferior to that in SBX AE-5. Also recently released external SBX G6 uses same design as AE-5. (includes Dolby Digital decoding for full console compatibility) Sennheiser GSX is again completely overpriced gaming hype product and should cost more like 50 $/€. Only good part in it is its HRTF algorithm. It uses low end Conexant DAC/amp chip (meant for cheap phones/tablets) which struggles to match level of budged price Audigy Fx/Xonar DGX! It might even struggle to drive HD600 serie Sennheisers. Sound BlasterX AE-5/G6 should cost 500+ $/€ by same pricing logic. Anyway for primarily gaming/movies music enjoyment tuned headphone wouldn't best best. Binaural cues don't like smoothing of treble/"warm" sound. That's not failure, but "feature" of C-Media chip used in many Asus sound cards. According to UniXonar faq Ironically Asus motherboards trigger it most often, also Win10 increases its occurrence.

Integrated sound cards have certainly come long way... From first not existing at all 20 years ago. Then being commonly interference magnets sucking interference from about anything, including someone farting in next block. Now with modern semiconductors they can push into human hearing's limits in blind testing: https://www.tomshardware.com/reviews/high-end-pc-audio,3733-19.html (sighted testing is scientifically invalid because of psychological factors) Though there seems to be lot of audiophile&marketing BS in them, with even most hyped ones in luxury priced motherboards suffering cases of interference. And when looking at what kind prices people pay without blinking eye from likely to get average in couple years graphics it's curious how little other side of gaming immersion interests. There are of course challenges/obstacles from physics of differing head shapes. But for closer to average head it's possible to get good directionality and even feel of distance with mathematical modeling of sound's/human hearing's behaviour and good headphones. Then again Microsoft themselves is major guilty in sidelining and regression of development of game sounds, which haven't advanced nowhere in last dozen years.

That marketing hype Craperia likely has completely overstrong bass, which with closed design buries other sounds to sea bottom. So any neutral bass headphones are certainly out of question and would be feeling like listening tin can in comparison. Do you have noisy environment or quiet environment? Open headphones basically leak environment's noises through unmuffled and you'll hear them normally when not listening anything. But they also isolate less heat, which good for comfort in long sessions. Closed DT770 would certainly qualify for strong bass, up to harmfull amount for distinguishing details in gaming. (to which it otherwise should be good) Same priced open DT990 Pro would be good for "balanced" fun gaming with good above neutral bass. As good thing about Beyer is them selling basically every part as spare part for DIY repairs if something wears.