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?

You need to get hold of the plate curves for the two tube types. That’s a graph displaying plate current vs plate voltage with separate curves for each bias setting displayed on that background. See how they compare.Then make some measurements of voltages in the amp itself. See if you’re in the ballpark for the 6550. The ambiguous response you got from Quad(?) was not super helpful. Or just try it.

Work on the signal tubes first.

The old production 12au7's you picked are on the bright/lean side per my experience.

The GE 12ax7's should be OK if they are drivers.

DeKay