Can’t use any other tube??!!?

No, can’t use just any 12AU7 in the "center" spot on that amp, as previously noted.

A few years back a member over at AA was trying the same and learned this directly from rogue "MUST be low noise. It doesn’t have to be an ECC802s - but what ever 12AU7 you put there must be super low noise." Referring to "tube rush" also discussed how some of the substitute military edition NOS Mullards are not all that quiet either and cannot be used in that center spot.

Go and buy a GE jan 12au7....upscale sells them. They are military grade and one of the quietest 12au7’s available. I must say that no matter, when using tubes as opposed to SS transistors, you will have more noise. It is unavoidable. That noise is only audible at a relatively non real world volume level, with no actual music playing. Once the music is playing, you are not going to hear the noise. I’m talking hiss....some hiss is normal. It can be reduced, but not completely eliminated.

It's a pre-amp section that is sensitive to noisy tubes. Very common. Call Upscale audio and they will set you up with some good choices.

^^^ this ^^^

new production mullards are not real old stock mullards, nothing to do with them other than the name, which has been purchased by the russian tube maker

you need real noise screening on the tubes, need to buy from someone who can do this... search other threads for old stock tube dealers - kevin deal is one of them, 4-5 others out there who do this properly

"...It can be reduced, but not completely eliminated..."

All 5 of my tube driven systems are dead quiet. YMMV

Any ideas why only the stock tube is working?

In any high performance tube equipment the tubes are hand-picked. Your 'replacements' are not. They are just random tubes. You need a 12AU7 that is picked for low noise and low microphonics.