I could not tell them apart at all. Based on everything I've read about tubes and tube amps, I found this to be incredibly surprising. I built an A/B switchbox for comparing source components and have run a couple of tests. I'm using a rockboxed iPod classic with FLAC files, and a pair of Sony SA-5000. The Valhalla has stock tubes.
I tried a variety of tracks in different Genres. Across all of them, I could discern no difference in sound what so ever. Has anyone else ever observed something similar from these two amps? Perhaps my hearing is not too discerning, but I have definitely heard tangible differences in other component tests with the same set up.
Is my test fundamentally flawed somehow? Is it possible that a similar configuration/layout of the innards is more meaningful than the fact that one uses tubes? (I have no idea if the rest of the circuitry is similar). Or maybe I'm just deaf :)