I have a Pioneer VSX-D810s (100W 6.1) reciever.
M60Ti mains are powered a Bryston 3B amp (120W) via preouts.
VP150 center is powered by a Thomas Patterson amp (120w) via preout.
Passive P.O.S. sub that is powered by the other channel on the TP amp via preout.
3xQS8 surrounds are connected to the Pioneer.

BTW the room is 24x36x9 with vaulted ceilings.

mains set to large
center set to small
surrounds set to small
receiver crossover set to 100Hz (can only set 100/150/200)
receiver configured to send LFE to mains (large speakers) also

The system sounds great, except when pushed.
The M60s strain with the lower bass, causing the Bryston to clip.

However, given the size of the room, I want so add a powered subwoofer.
So I was thinking of getting an EP350, EP500, or something like an SVS PCi 20/39. I would use the high level inputs, using the subwoofer to set the crossover at something like 60Hz for the mains.

When subs are configured like this, do you lose the output of the sub between the crossover point and the sub's higher effective frequency response?

Would I be better off setting the mains to small, and let the new powered sub(s) handle everthing below the crossover point (100Hz)? In this case it would be a shame to lose the sweet sound of the M60s below the crossover point and their lower effective frequency response.

Ideally, the best solution would be to replace the Pioneer with a reciever that would allow individual crossover points for each speaker and of course powered sub(s).