I may be wrong on this, but wouldn't the feed forward "correcting" amplification circuit in a thx amp cancel any distoration introduced by a tube preamp? In which case, you don't add anything from the tube preamp. It may be adding gain or not. It may be doing nothing at all.
I have searched and searched but can't find a satisfactory answer to this. I did find a very long thread elsewhere in which various different tube preamps were tried by lots of people and most of them noticed no difference in sound.
This makes sense if either the most difference is to be found in the tube output stage, which you can only get with a proper tube amp, or if any effects of the tube preamp are being nullified.