Hi,
When I bought my ATM3 (1996 and still working) they were delivered with AEG EL34 and Siemens 12AU /AX7 (rebadged Mullards in fact).
I replaced them after about 3000 hours by C SED, that were also very good. After that I found some Mullards XF 2 double O rings and mono ring getter. They did their job perfectly for many years.
I am back with AEG and very happy.
The OO ring getter xf2 were the best sounding ones without question.
The Mullards simple ring getter xf2 were second ones a bit before the AEG that are very good sounding and robust.
Many years ago the C SED were the best new production tubes and the price was fair.
Unfortunately they are not in production anymore and the AEG have become out of reach (12 tubes).
It seems the new production Russian Mullards are amongst the best for sweet sounding. It would be my choice if I had to choose new EL34 tubes for my Air Tight. Look also at the preamp tubes because they make a lot of difference also and they last longer.