Anyone familiar with Mullard 12AX7 10M tube ?

I had a pair that I used in a McIntosh MC275 about 10 years ago and I remember thinking to myself that they fell short of expectations.  Very small sample size... YMMV.

I see. Yes, it might be equipment dependent to a degree.

No!

If I may ask an additional question, I’m not trying to hijack your thread. Of the NOS Mullard 12AX7 tubes , is there much ( any? ) difference in the sound characteristics of the sound. I just picked up a pair of tube mono blocks and they each take 1 - 12AX7 and 2 - 12AU7’s. I’ve never rolled 12AX7’s but in rolling 12AU7’s I tried a standard Hammond labeled NOS Mullard 12AX7. Thanks, to the Op. Respectfully, Mike B. 

I can't say because I only installed Mullard 12AX7 narrow long plates in my VAC's Avatar SE preamp section and they sound excellent. I did read that, yes, there is a  difference, there is quite a range of Mullard sound.

A number of tube manufacturers used special designations for what they claimed to be higher quality examples within a tube family. My guess, at most, is that they may have factory tested and given this badge of honor. There is no way the 10Ms, Gold Brands of 5 Star tubes were made differently than their more common brethren.

The range in Mullard sound is based upon production periods in my experience. This is true of many tube manufacturers.

10M are sort of guaranteed for 10000 hours. Well, other Mullards might last just as long if you are lucky, but I don't count on it. I count on 5000 hours. So far I have about 2000 hours on the narrow long plates.

Well when these Mullards were made you didnt have components that really beat the tubes as some do now. All depends on the circuit I think.