Would the LFM-1 EX be overkill if used only for 2.1 using an RR2150 (no separate power amp) to drive main speakers which are very efficient Polk Monitor 70s? (The monitor 70s are quite loud only at the 9:00 position on my RR2150. They are rated for 275watts, known for their high efficiency, and each speaker has two 6.5 inch midrange, plus two 6.5 inch bass, and a 1 inch tweeter.)

Should I go with this sub since it's onsale or go down a couple sizes to the LFM-1 compact?

It's for a 3100 ft3 living room with carpeting and heavy drapes.

For a room that size I don't think the LFM-1 EX would be overkill.