Recommended 12au7/5814s for Atma-sphere MP-3

Just a couple comments about 12AU7 in general based on my experience in other gear...

As you have heard, the Siemens are clear and spacious. Crystalline.

Mullard 12AU7 (Blackburn production) have a slightly softer top end than the Siemens, and they have a slightly fuller and meatier midrange. The differences are subtle, but obvious. IMO, the Mullard are more "musical", the Siemens are more neutral.

RCA 5814A black plates are also quite clear and balanced. Neutral and balanced like the Siemens, but not quite as precisely defined in the treble region.

My favorite are the Mullards. I have put away five pairs. I still have one pair of RCA 5814A, and I sold my Siemens.

My hunch is that the Siemens in the Atma-Sphere combo is possibly too much of a good thing, and that the Mullards might bring some welcome coloration (albeit slight).

I am considering purchasing some Mullard 12AT7 for my MP-1 phono section.

My favorite is the 7316 sub for a 12AU7. I use them in my Jadis DA 60 but have rolled them into several different integrated amps with great outcomes. They are very good for upper end detail and have a soundstage that is unreal, in a good way. They are scarce and to my knowledge were only made around 1960 and only by Amperex Holland. The one caveat is that they can be very expensive these days. OTOH what good tube isn't? If you like the clarity of the Siemens for a strong clean signal try that other German tube Telefunken ECC82.
BTW as my old friend says quoting Kondo San nothing is unimportant. The 6SN7s do have an impact on the sound the Sylvania GTB IMHO are very nice somewhat warm tubes with a euphonic midrange. I won't write a novel about the available 6SN7s to try. They remain the only tube that is infinitely better as NOS or OS than anything new. Please note that I emphasize infinitely better. My own tube collection contains mainly older tubes. However new production has begun to climb up the lader as true NOS go extinct and pulls or throw away tubes are what appears to be what is being sold.

The Mullard is the one obvious pair I do not currently own that I do want to try. Are they all the same (CV4003 and the 12Au7s), any good sources? Does seem like Andy at VTS carries them. I agree that the Amperex would be great to own, but the prices and scarcity makes give up on them. The Mullard's seem to be more accessible. If I could find some strong matched GTBs I would also give them a try, if they are warmer than the GTBs it might work well with the Siemens.