Amperex 7dj8, Siemens e288cc like mentioned above, and the late model single getter, no shield 7308 Siemens. These all lasted 2 years in my previous m3a. The 7dj8 Amperex really sounded killer, as did some Toshiba 7dj8's (sounded similar to Mullard cv2493). I still have a few of each if interested (the NOS sleeves of 5).
The key to the 7dj8/PCC88 is to have a pair in use with a pair of regular Russian 6h23 or the Siemens single getter 7308 tubes. In the M3a the tubes are run in-series, so by having a pair of 7dj8 in the line stage, and 7308 in the phono, the extra voltage going to the 7dj8 lowers the filament voltage seen on the 6922/7308 tubes (so they last a LOT longer). Really does work. I had my M3 then M3a for quite a few years, and they lasted a long time in this confiquration.