The early Phase Linear amplifiers used RCA 410 Driver Transistors. Later units saw RCA 66546 Driver Transistors, which are the exact same device with a newer RCA part number. These are the only recommended drivers for a Quasi-complementary (in which all NPN power transistors are the same - PL909, XPL909, MJ15024; or the upgraded MJ21196G).
If you need some driver transistors, most rebuild techs on the Forum have some.