Can’t use any other tube??!!?

It can be done!  I tried dozens of preamp tubes on my old CM Ii.  Excellent sounding amp that upgrades nicely with small signal tubes. 
See Brent Jesee Audiotubes.com.  Spec  low noise screening for an extra 10 bucks.
My favorite was Amperex NOS.  Buy an off label Amperex and save $

Agreed.... my preamp is very sensitive to noisy tubes.   I have a JJ 6922 in there now.   Just put a brand new Gold Lion in,  noise.   The Gold Lion tube is dead quiet in amp but rush in pre. .   So i put the JJ back in.   Sounds very good,  not quite as good as the GL...still sounds really good, just different .   Im going to order a super low noise tube .....

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.

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

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

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

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.

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.

"...No I'm just joking. BUT the valve (tube) tester will tell most of the story..."

In high gain pre-amps good tubes can be noisy and they have to be specially selected for low noise. Good shops like Upscale will use special equipment just for this.

Upscale Says: " After this test, the tube is listened to with high-quality Sennheiser headphones in a circuit that simulates a phonostage. In other words, real-world voltages and high amounts of gain. This lets us hear any noise that might be present in a tube, as well as grading the level of microphonics present. For a deeper discussion on the ins and outs of microphony..."

OP you know anyone with a valve tester?

It's pretty simple you have a piece of equipment that only one valve in the world will work, just ONE.. I hope they or IT doesn't go bad..:-)

No I'm just joking. BUT the valve (tube) tester will tell most of the story.

You have some noisy valves and BAD contact could be some if not all of your problem.. Just because they sound OK in one unit may just mean you have better contact on the pins, from one to the other..

Does that happen very often? Ask a bench tech that works on valve amps A LOT. 

101 valve maintenance. Check the valve pins and SOCKETS for cleanliness. Tighten and clean the pin pockets AS NEEDED...

Take the questionable valve, a wad of cotton and a little WD40 and spray the cotton ball with wd40. Plunge the pins several times into the cotton.
Be aware of the silk screening on some old valves. The WD40 might loosen or stain the printing.

IF that doesn't help, which it will no matter the change. It is always a good idea to start from the side of "I checked it correctly" and that cleaning really made a difference..

The long and short of it is. 

First clean the valve and pockets and tighten them if needed

Second, test the valves you have OR buy a pair of noise free valves from a good supplier.  Tell the supplier what your problem is.. NOISE.

Just so you know transportation of valves is NEVER guaranteed. If they bounced across the US and were thrown on the front porch..

TWO week is a wise mans approach.. Keep an eye out.. BUT usually the first 2 minutes to the last of 24 hours is a good sign they survived the transportation experience AGAIN. They will probably have a long life..

Happy hunting..

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.