Set them to what ever sounds best, some even run the sub with the M80s set to large for a double bass effect to help smooth out the in room response.
I perfer the Mains M80 to large, anything over 45ish is going to fade and roll of anyways. Also, it seems to blend in well between the sub and M80. the volumn level of 30hz and below on the 80's is pretty low anyways.
Unless your amp setup is at 80hz for the fronts/Large and you have the sub set to set to LFE and not LFE + Mains. Then again if you have the mains set to small and you have M80's that's a waste, that's like buying a Cadillac and driving the Pinto or better yet you drive the Pinto and the wife drives the Caddy.
Too each his own preference ...
