I have owned both the 8417 and silver 90's but am not familiar with the 100's. The Silver 90's were spec'ed to be biased quite high-100ma which puts them at about a Class A design, I think (I'm no techie). I ran mine at 80ma to save tubes and didn't notice any sonic degradation. Quicksilver estimates tube life to be 1000 hours and I have found this to be fairly accurate.
Quicksilver Mono 100's
I am considering the Quicksilver Mono 100's as amps for my Quad 988's. However, there is something I don't really understand about these amps. The amps produce 100 watts per side from only two KT88 or 6550 tubes. The total tube compliment is: 1 12AX7 input, 1 12BH7 driver, and the above mentioned output tubes. I don't understand how they get 100 watts out of just these two output tubes. Are they cooking those tubes with high voltage? Running them into class B? What are the implications for the sound of this amp? Has anyone spent time auditioning it? Thanks.