I don't recommend what you're trying to do, however, if you go into System Setup, Manual SP Setup, and then Speaker Settings, change the subwoofer to Plus. This sends all bass at the frequency cutoff you specify from all channels regardless of whether the speaker is set to large or small.
I have M80's and they are set to small. Frequencies below 80Hz are not directional. So you honestly should not notice more bass sending it to the M60's and the sub at the same time. Depending on speaker placement, you actually could end up making it sound like there's less bass due to frequency cancellations.
You can experiment with this and see what happens and ultimately decide on which you like best. Far as I'm concerned, though, the EP500 should be plenty and will handle the lower frequencies much better than the M60 (or M80 in my case). I have an EP500 as well.
BTW, I have an 84, so I'm assuming that the 72 has similar menu options. If you can find the speaker setting for the Subwoofer that's what you're looking for.
