Diagnosing Distortion in PC Based System

I am experimenting with a new desktop system:

* iTunes, Apple Lossless, and WAV ripped through iTunes
* WAV files ripped from EAC and Foobar
* Sony Vaio laptop with Windows XP
* Cheapo USB interface by Creative Labs
* Optical output to Behringer Ultracurve Pro 2496
* Also, RCA output to Musical Fidelity tube buffer
* Balanced output to 2 Behringer amps, bridged as monoblocks
* ATC SCM 7 monitors in "nearfield" configuration

At first, I was really thrilled with this little system and stayed up late into the night listening and having fun with both music and movies.

After a weeks break, however, I tried it out last night with less success.

Although the mid bass and midrange are still warm and surprisingly full, and the treble on the ATCs is mostly smooth and non fatiguing, I noticed a very audible, very brittle distortion somewhere in the treble.

This distortion was was particuarly noticeable on piano attack. As the ATCs are supposed to be unusually adept at piano, this was disappointing and quickly become very irritating.

At $199 each, I shouldn't expect miracles from the amps, but they are claimed to have low distortion and with 500 watts each while operating as monoblocks, I can't imagine I am hearing clipping?

The EQ, although also bargain priced, has been praised and claimed as quiet by other users on Audiogon.

Is is possible this could relate to the Apple WAV rips vs the EAC WAV rips?

And God forbid my Chinese amps damaged my tweeters as someone suggested in another thread? (Have not been cranking heavy metal on them or anything.)

How would you diagnose and isolate this distortion, without schlepping my Levinson amp on top of my desk and rewiring everything?

Thanks for any thoughts.