elira

The HD6XX + Schiit stack is a solid option unless you don't like the sound signature. For $400 I would get a Grado SR325e, for a closed back without amp I would get the ESS 422H ($200). Also the Sennheiser HD 660s may be obtainable under 400 second hand, and those sound good without amp. Also consider that the Schiit stack ($200) is a solid amp/DAC that is going to work fine with most headphones.

Fulla 2 is a good option http://www.schiit.com/products/fulla-2

Ok is very subjective but those speakers should be ok for most people.

Horns are very efficient.

This thing https://www.hidizs.net/collections/accessories/products/dac-cable You could probably find something cheaper somewhere else. like https://www.amazon.com/dp/B00IRVQ0F8

If you are in the US the ESS 422H are nice, and easy to drive.

Are you using a custom crossover?

I think it’s worth it.

Also check this one K&M 16310 https://www.amazon.com/dp/B005MZKCJM/ref=cm_sw_r_cp_api_i_KqqbBb5VZ2FJ6

I like this one https://www.amazon.com/dp/B01DNLLTIE/ref=cm_sw_r_cp_api_i_GnqbBbVBKR619

Apparently yes, the t60rp are like modded t50rp. I don’t have first hand experience though.

Here you can find parts https://critesspeakers.com/ not precisely for lascalla but for other klipsch speakers.

The Mobius are very promising, I tried a pair and it felt like a good pair of headphones, obviously it's still in pre-order but I think they will be really good. The MDR1000X would be a safer bet, also the Noise Canceling is useful in airplanes or noisy environments.

Take a look at the Audeze Mobius.

The Sony WH-1000XM2 are good and have noise canceling (The first version is cheaper). The Audeze Mobius seem like a good option https://www.indiegogo.com/projects/mobius-immersive-cinematic-3d-audio-headphone-headphones#/ (Hurry before the campaign ends, otherwise the price would be much higher). PSB M4U 8 Wireless are also good option (A little over your budget). I personally would go for the Mobius.