I think it's mostly trying to match efficiency & power handling but not sure -- no sense having "mighty towers" if the surrounds can't keep up. If you're going for 60s more for frequency response/bass/sound quality then matching with 4s might work well.

This is probably a "call the nice folks at Axiom" question.

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