Hello Paul,
sorry for the late answer. When I bought mine, 20 years ago, AT didn't made or branded his own tubes.
Before get the funds for buying mines, I saw some arrived at my prefered hi fi shop with Sovteks, some equiped with Mullards from the last generation (unfortunately not the best sounding) and many many equipped with AEG.
Mine were with AEG as a lot after that because AT bought at that time many new unused AEG EL34 speaking. 12ax7 and 12 au7 were Mullards branded Siemens.
For EL 34 the best sounding tubes (in my system and for my tastes) were Mullards XF2 OO. They can be branded in either Mullard, Siemens, Miniwatt, Valvo, Amperex...etc. they are all made in Blackburn Mullard factory if the acid code is B (for Blackburn followed by a number for the year a letter for the month and anither number for the week of the month they were made.
AEG work great but don't have the force and finesse of the Mullards. C SED were good for the medium but not as detailed as AEG. The Siemens (made in ex Yugoslavia seems a bit harsh).
AEG were made in East Germany by RFT. RFT was building his own tubes and also for many others (Telefunken, Siemens, AEG, Haltron etc.). It seems the tolerances were not the same for all but I don't have evidence for that.
The little tubes 12 au and 12 ax I tried many and it appears you can tweak the sound. At that time I use Tesla E83cc (same as ecc803s telef) with CIFTE WAH 5* EAMR (établissement armé du matériel radar).
The choice is very dependant also with the tubes you have in the pre. It's been very long to adjust for me. I've been hleped by a friend of mine that had the same ATM3, so we have compared, talked about exchanged tubes and advices.