Do the change on the sub. Leave the receiver crossover at 80 hz. Also check the "Bass Out" setting on the receiver and make sure it is set to "BOTH" (you have three choices -- SWFR, FRONT, BOTH). This will make sure that the low frequencies from surround and center go to the M50s AND the EP125 so they can share the load.

[EDIT] I downloaded a manual for the 550, can you tell ?


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