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

Flabby bass is about damping factor as much as anything else. Don’t expect a tube  amplifier to drive a 4 ohm woofer without “flab”.

Phase splitter is usually done by driver tube and is better done by a 12AU7 than by a 12AX7, but anything is possible. A tube that has cathode and plate resistors of equal value, classically 47K ohms, is the phase splitter. But there are other ways to do it too.

12AU7 is probably used as a driver tube, which means it adds no gain and probably has little effect on SQ.  However, in my experience, there is no such thing as a good sounding 12AU7.  I replace them with 12FQ7 or 6FQ7 (depending on the available filament voltage), which sound noticeably better. If you're swapping power tubes willy nilly, then you need to be aware of how the amplifier can run one tube vs another.  If the plate voltage, grid bias, etc, are inappropriate for one tube vs another, then that is going to affect how you judge the tube. Ideally you would want to reconfigure the amplifier to run the power tube under conditions that optimize its performance, before you judge the tube. That usually can be done merely by changing a few resistors after taking measurements of voltages.

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.