By no means are my methods of testing perfect and I used very basic means to test all of this.
I used ROON(exclusive mode)> PC> USB> DAC.
Used for testing was a S.M.S.L SU-9 as a DAC and AMPS used were CAYIN IHA-6, REBELAMP, S.M.S.L SH-9 and A&S MOGWAI SE.
HP Used were Focal Radiance and LCD-5.
I did similar testing a year a go with SS AMPS and a few DACS like the topping A90 AND D90 Stack and other SS Amps and a little dot mk2 and posted similar results on a facebook headphone group and was bombarded with comments like:
ITS YOUR USB CABLES, YOU HEADPHONES ARENT REVEALING ENOUGH, YOUR POWER SUPPLY MUST BE AFFECTING THE SOUND, YOU NEED TO CLEAN YOUR EARS ETC ETC ETC
I since tried better USB cables and a more expensive power brick, more expensive power cables but these did nothing to the results.
My current pc motherboard has even got a specific ISOLATED USB SLOT FOR DACS. This should make no difference anyway but thought I would mention it.
I know my power is clean as the house was built about 7 years a go and I get no noise from my gear.
Not even the Mogwai SE.
Unless I use the Radiance and go to high with the volume I get a very slight hiss.
Songs I used are to many to list. Easily more than 100 songs but mostly EDM as this is what I listen to the most and know how it should sound but also but rock, jazz, heavy etc.
I am just shocked of how similar all my amps sound. Specially the SS stuff as the difference is not perceivable from my ears when switching across from one amp to the next wen volume matched.
The Mogwai SE is obviously different sounding but still not as much as what I thought.
I must say it does change depending on what tubes I am using.
So far the stock tubes 6l6gc sound the closest to my SS amps just with a slight roll off in the treble from what I can detect but with the KT88 fitted there is definitely more change is the sound and more enjoyable to me as the hole reason I wanted the Mogwai SE was for is (different sound) to SS Amps.
I feel like the bass is thicker and slamier. But I need to do more testing to this as it takes a lot of time.
So my experience so far is as long as your amp has enough power and is a good impedance match for your headphone there is no reason to go to expensive on this sort of stuff. Unless you go high end tube amp like a Mogwai SE for example and want a very specific flavor for your headphones.
I strongly agree with Resolve when he says spend most of your money on headphones as these do make a massive difference on how sound/music is presented. Amps and Dacs are not as important as most reviewers make out them to be.
Example of this is when I go from HD600 to a Arya the difference in how my music is presented and how it sounds to me is absolutely massive. When I go from amp A to B the change is so so small if any I personally can’t tell a difference.
Hope this helps.