Just a little update. I spent some time with both sound cards today, using redbook cds and taking the Go-Vibe out of the picture to simplify things. I also replaced the A250 with an AKG K400, which was the pick of the AKG range before the K501 came out--they're practically identical except the 400 is black and has very deep bass (it actually has a broader reported frequency response than the 501, 15-32k vs. 16-30k, but I bet in practice the detail is better on the K501). I chose this headphone because it is very warm-sounding, thinking that it was unfair to pair the dry, bright character of the Echo with dry, bright headphones like the A250.
The first thing I was able to determine was that bitrates were not an issue. Yes, you could sometimes hear a bit of difference between the mp3 and wav files, but the character of the cards was the same either way.
Now, the K400 did have the effect I was looking for: it paired much better with the Echo than the other headphones did. For about a half hour I convinced myself that the Echo had a bit more detail; it certainly had a crisper-sounding response on complex passages. I was starting to think that this was really just a battle between a warm dac and a bright one, a 'fun' option and an analytical one, with the analytical Echo being a bit more capable than the fun TB. But somehow the vocal presentation wasn't quite right, and I couldn't make up my mind. Then for variety I tried some Balkan dance music, and I realized why I just can't bring myself to like the Echo.
It's pace, rhythm, and timing. The TBAAM for all its limitations still has a healthy dollop of PRAT, and the Echo just doesn't. You can't tap your foot to it, it's like dancing with a robot. Yes, I can find musical passages that it can handle better than the TBAAM, but in the end I just toss it into its box again and plug the audio advantage micro back into the Go-Vibe.
So, then, I need to find a different upgrade path. I was toying with trying a Headphonia, as Lini suggested above, but I'm expecting a much better amp in a couple of months (custom build), so I'm thinking of just focusing on a cheapish dac. I guess my next step is to read up on the Silverstone and maybe the Fubar, now that I have a horror of PRATless dacs. Please pass on any suggestions, and either way I'll be sure to report back with comparisons next time I switch sources.