Pass Labs xa - .5 amps

To be more exact, the result was a tonal imbalance with the highs louder than the bass and midbass, resulting in a tipped up or bright sound. The tube amps used were VAC MusicBloc 160 push pull and Atma-Sphere M-60 OTL.

Physically that might be explained by the higher output impedance of the tube amps. As output impedance nears that of the speaker then you will modulate the amount of voltage reaching the voice coil by the variation of the speaker impedance with frequency. For example, if the amp has an output impedance of 4 ohms then where the impedance of the speaker drops to 4 ohms you will get roughly half the voltage at the voice coil than you get at a point where the speaker impedance rises to 40 ohms (at 4 ohms half the voltage is dropped across the output of the amp while at 40 ohms only 10% is dropped). Obviously, if the speaker has been designed to be flat in frequency response when driven by an SS amp then you will hear a "tilted" presentation when played with a higher output impedance tube amp.

I am sure Tvad knows this very well - having directly experienced it and based on all his other knowledgeable posts here on Audiogon. So my comments are simply intended for those who are puzzled as to the cause of Tvad's observations...that's all.

Tvad, I was doing a search on the silverlines, (I've always wanted to hear them) and I saw several mentions where Alan Yun preferred tube amps. It might have been for a different model. Regardless, it sounds like you've sorted it out to your liking, and after all said and done that's the important part.
Just because the Pass gear didn't work for us doesn't mean it won't for somebody else. My new mantra: Listen for yourselves, people!