I wouldn't pair such quality speakers as the M80s with a sub like this. I would either stay without one or buy a good one.
But I still think that what you're looking for is the the 50/80Hz area that the M80s play just fine (flat). If you get a good flat sub and calibrate it properly, you shouldn't get more bass from the sub than from the M80s in this frequency range unless you run the sub hot a few dBs.

Bruno
M80s/VP180/QS8s/EP600/AVR-890
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"The problem is choice..."