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

All modern-ish Rogue tube amps use a 12AU7 tube as driver per push-pull pair of KT tubes. The Apollo and Zeus used a pair of 12AU7 to drive fully three push-pull pairs of KT. I agree with the asssement that 12AU7 is not a great tube. I definitely prefer push-pull amps that use 6SN7 drivers, or really anything else. 

But due to the fact it’s perhaps a bit "under-powered" in this driver role, it does make a noticeable difference to tube roll these slots. Similar-type subs are a good choice for most effect. 12BH7 is a sub I’ve used many times - it’s much more dynamic, detailed, and punchy than a 12AU7, though to be fair the Rogue amps were already strong there. The 12AU7 will give a more "homogenous" sound - maybe preferable to some ears and systems. I didn’t try other type subs. Note a 12BH7 eats up more heater curent - 0.6A per sub versus 0.3A - but when each KT tube is sucking down 1.5A - 1.8A, the extra A’s from a couple 12BH7 are unlikely to cause issue. Rogue (Mark) used to bless this sub, though they might have clammed up on this more recently. I used it for years with no issue.

The extra center 12AU7 on Cronus is an input / gain slot. Good to roll there. 12BH7’s tends to be noise and microphonic so it’s best to keep them at the downstream driver slots.

If you do try 12BH7 - Sylvanias and Tung-Sols are very clean, detailed, and punchy (Tung-Sols tending a bit more toward bright); RCA’s are warmer and richer. 

No, leave the GE 12ax7's as they should be good and long lasting.

The single (center?) 12au7 is the one I would experiment with as it should have the most affect on sound  and then perhaps experiment with the 2 12au7 drivers if not satisfied.

I do not have a problem with 12au7 tubes and have been using them in gain positions for 20+ years in my preamp and in vintage power and integrated amps.

I've yet to listen to an amp using 6SN7's that I've cared for and the same goes for Audio Research and BAT amps using the "super" tube - so each to their own.

Look for a single well testing Mullard, Amperex or RCA 12au7 with balanced sections (it need not be a super duper expensive version or be NOS - just avoid the later dimpled disc getter Amperex as I previously noted).

DeKay

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.

Also, have you tried a lower bias setting on the power tubes you are using?

Doing so (within reason) may move you further towards the sound you are seeking.

Try it for free before shopping for alternative signal/power tubes.

DeKay

PS:

I’m a big ST-70 fan.

It’s always subjective, and every tube behaves a bit differently in each circuit, but I’m finding the Tung Sol 7581A (a 6L6 variant) sounds superb in my Dyna/VTA 70s. I’ve run a few EL34s, and I really liked the Gold Lion KT66, but have been absolutely thrilled with the musicality, clarity, and texture the 7581As produce, along with 12BH7a driver tubes in these amps. Best of all, they’re not super expensive.