I believe everyone on this thread would be in total agreement that Mr. Nelson Pass has to be one of the greatest designers in the history of modern high end audio.
I now power my bi-amped system with a pair of Pass Labs XA-100 monoblocks and a pair of Threshold SA-1 monoblocks. I do believe that both pairs of amps are the best sounding designs in either the history of Threshold or Pass Labs. I used to have a pair of Pass Labs Aleph 1's, which are a classic design also, but believe the new XA amps have surpassed the sonic performance of the Aleph and SA series amps.
Unlike Lawyerman, I find the XA-100's to be a better sounding amp then my SA-1's in my system, however they are both great and its amazing that the SA-1's are still almost as good as any SS amps on todays market considering that that they are twenty years old!