Hi MCL,
Yes, that is correct thinking. When you set the M60s to Small and the crossover to 80 Hz, your M60s will still produce some bass, but as frequencies get lower below 80 Hz, the suwoofer will gradually take over the low bass duties that it's better equipped to handle. Given the size of the room, I'd suggest the Axiom EP350 which will deliver really deep powerful high-impact bass in that size of room. If you're not budget-restricted, the EP500 goes a bit deeper and plays even louder, but the differences between the two are not dramatic.
You can experiment with the crossover frequency for the sub--try 60 Hz as well--as well as the Small or Large setting for the mains. Some listeners like the "double-bass" effect of setting the fronts to Large and routing bass to the sub as well. In my room, I get too much bass using that setting.
Regards,
