I don't even know if I wanna join this.... fuck it.
Onboard audio is good. Seriously good. the Realtek series is good, ALC 889, 892, 898, 1150, all of those are actually great. the DAC's have good specs, although some have slightly high noise floors. the small amps are transparent, and have low output impedance. Granted, they don't have a lot of power, but it should be just fine on sensitive / low impedance headphones. I myself very rarely go past 30% with my ALC892, using Shure SRH440, Pioneer SE-A1000, CHC Silverado or very rarely the Audio-Technica ATH-M50S. with IEM's (JBL J33 and Soundmagic PL50) I rarely go past 15%, so it definitely gets loud enough. Haven't had the chance to use less efficient headphones though, such as the 250 ohm Beyers, or the Sennheiser HD600 or 650, although I've heard numerous reports of them working just fine on onboard audio.
Objectively, if an amp changes the actual sound instead of just the volume, it's a bad amp. Some might like it, but it's better to go with a transparent solution and let the headphones do the talking, or use an EQ. I'm willing to bet that you can EQ your way to make it sound like a tube amp. I do actually have a soundcard in my PC, an ASUS Xonar D2X, which I use for speakers right now because I can't be bothered using them for headphones as onboard sounds exactly the same, and doesn't break IEM's like my D2X did with my first pair of JBL J33's. that's the high output impedance for ya.