All of Telefunken NOS 12au7 works great with M1200, I think. I love the “Mazda” brand of Telefunken. I also tried Telefunken Rib-plate NOS. Both works great and yours too, I am sure!
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Another Telefunken Mazda happy owner here, thanks to @dchang05. The sound is open and organic even if warm at the right point like my favorites Gold Lions, that are pleasant too but a little bit too much on the warm side.
Telefunken Mazda Diamond Bottom Smooth Plate tubes are definitely my M1200 sweet spot. It’s wonderful when little parts can give you a sort of perfection accordingly to personal tastes.
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Recently I acquired a pair of NOS Telefunken 12au7 smooth plates. I was curious to see how they compare to the NOS Telefunken “Mazda” brand that we like. After one day of settling in, both are superior to any other tube I have tried in M1200 so far. However, Mazda sounds less analytical and gave a bit more luxurious mid-range to my liking. In another system one may reach a different conclusion.
Luca, now you do not need to try other types of Telefunken NOS 
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The M1200s use 12AU7s. The 6DJ8/6922/ECC88/E88CC is NOT interchangable with the 12AU7.
Thanks, you are my trusted tube tester! I’ll give them a try for sure, even if not soon.
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Yeah, I was rolling BHK tubes to 6 volts and probably got confused somewhere or something, but hat’s a really old post! Haha!
Where’s a trusty deal on my next M1200 tubes today? Anybody make a recent purchase? I want clarity over warmth because my system is borderline too warm with 40.2 Harbeths and 6 NOS Mullards in the Hagerman phono stage.
May want to consider the RCA Clear Top 12AU7
Clear, detailed and lively when installed in my preamp. While I do not run the 1200’s my system is just on the warm side of neutral overall and these tubes give a little jolt of energy when I’m in the mood
Think I paid $99 ea. at Tube Depot
Best,
-JP
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Each tube type may age and breakdown differently, but the original PSVanes 12AU7 that came with my BHK Preamp began hissing and popping from one speaker and whinning from the BHK unit itself after 1 year of 3-4 hrs use/ week. I switched the tubes and the hissing/popping went to the opposite speaker. I plugged my backup pair of PSVanes and problem solved.
I appreciate all the inputs regarding sound benefits of using Gold Lion 12AU7s cause I need another backup pair. Not sure though of the benefit of cryotreating them. I may just stick them into the M1200 instead of the BHK Preamp since you guys seem to prefer it this way. Perhaps I’ll also enjoy the same setup.
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I noticed a blue light internally in one of my M1200s. It stayed lit even when I switched the amp in stand by mode.
So I decided to replace the tubes this evening, dang!
Never seen a broken tube?
@dchang05 my first pair of NOS Telefunken Mazda lasted 4 months. This pair lasted 7 months. Now I’m using my only pair I had as spare so I need to order another matched pair to Brent this week. Definitely not lucky with tubes!
The good thing is that, since I replaced the tubes the sound is really glorious, I’m so impressed (again) how these tubes could sound with M1200s. Really stunning and silent, too.
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Oh! I have never seen that on a tube! I just replaced my GL in Pre, and I decided to keep using the old Telefunken “Mazda” because they still sounded great at 2500 hours (I compared a new pair of Mazda and the SQ is the same, there’s no loss on the old ones at all).
Your Italian 230V may have something to do with it. Or, you are listening at a much higher volume than me.
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I love low level volume, 55-65 dB range. I’m eating tubes like ice creams here, GN on BHK Pre every 10-12 months, NOS Telefunken Mazda on M1200s every season.
If current is the fuel of an HiFi system, well tubes seem to be IME tires I need to change from winter to summer.
I guess your system is like an Italian sports car, it burns rubbers (tubes) like crazy! 
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blue light in the tube doesn’t 100% mean it needs to be replaced
the best way is to find somebody who can measure the tube and maybe there are a lot of life in it
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@jamesh: what does it mean when the internal blue led light stays on, then? Even after the M1200 has been switched in stand mode pressing the front button.
One amp has the light on (no click sound), while on the other amp the light goes down after a few second (with an audible click sound).
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He is referring to the blue LED next to the tube which shines when the unit is on and operating - not a blue glow from within the tube.
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ahhh sorry
But you are correct, blue light in a tube is fine. 
The tubes in the M1200 should last a lot longer than what you are experiencing, it’s an input stage, should last many thousands of hours.
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I put these in a month or three ago. They are to soon be placed in the BHK pre.
If anybody notices a blue tube light staying lit in hibernation, I have a few ideas for ya…
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That blue LED is purely aesthetic. It’s been quite a while and I don’t remember what would cause one to stay on. For some reason my memory is telling me it could be the trigger connection. I’m probably wrong though.