STF-2 is definitely a good sub.

In terms of pure frequency response I would still take M2 over M3, although the difference is pretty small. On the other hand, although I haven't actually tried it I'm pretty sure that M3s would do a better job of filling what is a relatively large room when paired with the same sub, so...

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8