Good news: since they’re going in an amp, not a preamp or phono, you have more options for 12au7 tubes & subs that would be too noisy / microphonic in those slots.
I’ve recently been using RCA 5963 (close 12au7 sub) in an amp - they’re affordable (beasue 5963 was not intended as an audio tube) and have a nice balanced sound. There are gray and black plate variants. They’re too microphonic for preamp / phono, but fine in an amp.
I’ve also liked GE 5814 (triple mica, gray plates), as a warm 12au7 sub that’s not quite as "wooly" as Mullard. Also affordable. 5814 is a very close 12au7 sub.
If your component’s PSU has a little extra heater current to spare, you can sub in 12BH7 (very close sub except for 0.6A heater current draw instead of 0.3A per tube). These tubes tend to be more dynamic sounding than 12au7. A power amp is more likely to have the heater current to spare, since the power tubes take up much more of it than small tubes. I’ve subbed them in a few different amps.
6189 is another sub you can do (12au7 equivalent). The Sylvanias and Philips ECG of this tube tend to be more "solid state" like in their sound. Cifte / Mazda / France made some silver plate 6189 that are very good, definitely not on the warm side though. Siemens also made some silver (nickel) plate 12au7 some people like - man, mine are microphonic as hell though - should be OK in an amp.
RCA cleartop 12au7 are everywhere (often rebranded - millions were made) and some people like them. They used to be pretty cheap. I find them "fine" but nothing really exciting.
I haven't tried Brimar or Amperex of 12au7 types - but based on their 12ax7, they should be slotting between Tele and Mullard sound, like you are looking for!
