time to discuss kt-88's

Tubes taking 400 hrs. to burn in? That is completely ridiculous. Electrons traveling in a vacuum don't "burn in." If you argue that cathode & anode burn in, that is going to happen within a few minutes from new. You're probably just getting use to the sound of the tubes, or they are changing in sound because they have already aged in 400 hours.

As far as comparing KT-88s, this will differ from amp to amp. You need to get recommendations from Quicksilver owners, or from Quicksilver themselves. In the ARC VT-100 Mk II, I found the Svetlana to have a beguiling sound with some pleasant 'tube fog' in the midrange. Decent, but not outstanding bass, silky treble that does not have the ultimate in detail however. The EH KT-88 is a very good tube, as is the JJ KT-88. I would try those in the Quicksilvers, as Quicksilvers have had a reputation for a very midrangy, bloomy sound that could use some bass tightening and high end extension.

I wouldn't want to have tubes that take over 400 hours to burn. Based on an estimated useful life of 1,500 hrs., 1/3 of the tubes' useful life will be gone before it is burned in.

I have heard claims about how long a tube is supposed to last before. When I had small signal EI tubes (EL84) their construction appeared very cheap and unimpressive. The glass was thin, the pins were flimsy. Overall, they did not inspire confidence. Actually, I remember replacing them with EH tubes, and the sound was far better. I know we're talking about KT90s, not EL84. However, I would expect that if the construction and reliability were not good on other EI tubes, these would be questionable. I remember many criticisms of EI-90's blowing up amps. In fact, I owned an amp that the prior owner had to repair because of failing EI KT-90s. I hope you are right, and these present production KT-90s are in another league.