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

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 guess to more directly address your question - exactly who said EL34 would have less lifespan here, unless you’re purposely biasing to run hot? That’s the first ever I’ve heard this. The bias on these amps is infintely adjustible per tube, and an EL34 can handle much more plate voltage than these amps throw at it. In fact the original Cronus and Atlas shipped with EL34 (I had an Atlas like this - it is a Cronus without the 12AU7 "preamp" tube and volume control).

As for tube sonics - Tung-Sol KT120’s give a perception of more powerful bass. Its midrange can be a little drier than with Gold Lion KT88, so probably not the right move for what you want. 6550 should not be a problem in these amps - very close to KT88. I’ve even used Tung-Sol 7581A (very close to a 6L6GC, not that close to 6550 / KT88) in Rogue amps - you’re probably approaching its plate voltage limits, but it worked in the ST100 and gave a bit of a more "classic" tube sound versus KT88 / KT100. A nice tube in the ST100, at least. Been years since I ran my old Atlas with British EL34, but that was pretty decent. The Mullards do what they can on these amps - they will never be "warm"-warm. You put those same Mullards in an Eico EF-87 and you get WARM-warm. 

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.