KT88 vs 6550 vs ? for warm midrange - using Cronos Mag lll

RCA clear top 12AU7 and GE short plate 12AX7s are pretty pedestrian tubes and not known to be warm in fact a bit the opposite. So before changing power tubes. you should research small signal tubes which tend toward the warm or rich and start at that point. I have always found that the tubes feeding the power tubes have more impact on the sound. 

Rogue amps and "classic" vintage tube amps sound are on 2 opposite ends of the spectrum. Rogue's formula is a punchy, fun sound with great bass and lots of tube Watts per dollar. They're going to be very reticent towards rendering a plump, lush midrange - you can only push it so far. If you have a Cronus / Atlas with EL34 (British Mullard) and good NOS small tubes (not later 1970 / 80s Philips ECG / GE), then I'd say that's about as much as you can push Rogue towards the classic tube sound. 

Be a bit wary of small signal tube rolling. It's fun but a lot of the "wow" is from the inital dopamine hit from a initial change. No need to go on a frenzy buying up all the vintage makes (I regret the time & money I spent doing so). You're not gonna make a Rogue sound like a vintage tube amp, in the long run. 

I recently managed to get hold of some Svetlana (St Petersburg) KT-88s from valvesnmore.com. They are at least as good as the Sophia Electric tubes they replaced. But I too, have read good things about NOS winged C 6550 tubes.

I'm using Quad II/forty monoblocks, but I am not sure if they are fixed or auto-bias (looking at a circuit diagram I believe it is fixed bias). Quad have told me the 6550 tubes will work, but "since the amp is designed for KT-88 tubes it may not be optimal." Apart from slightly reduced power output (35W vs 40W) the real issue is bias. Am I right to think that the fixed bias that can cope with any number of KT-88s that might be used is also likely to cope just as well with 6550s?