the way I can usually tell is because when I hook things up this way, my better quality lossless files will actually sound worse than my lower quality MP3 or below quality tracks. When I have a clean signal path from the source into the amp inputs, the lossless tracks clearly sound better than my lesser quality tracks. I'll get this ironed out, though, and report back. I do really like what the tubes did for the low end, though...very nice sounding.
It's good news it's performing well at the low end. The high can be difficult to deal with. I've found a recording that can produce plenty of sibilance and I've been experimenting with different dampening materials to take that edge off. This approach is probably already well known, but I got some promising results using thin foam covering the center of the driver. A quick easy test is to find a good reference track, then just use different materials in the ear cups. Is this called foam rolling? lol.