Right now I have 3 Ascend-1's for the front and 2 HTM-200 SE for the rear. Powered by a Onkyo TX-SR304. I normally listen with the receiver set at half volume.

I am interested in getting the Rythmik F12G 300WRMS amplifier subwoofer.

What subs should I compare with it? 80% music 20% movies. Under $700 shipped.

The room is 16' x 13'.

The two other sealed subs of that size and price range are the SVS SB12 Plus and Epik Vanquish. Only the Rythmik is servo and in my room which is comparable to yours 16x12x8 was flat down to 13hz. The other two will start to roll off well above that with the Vanquish probably being the closest.

For music which is mostly your preference I found it to be outstanding and never had any issues with output. For HT I found it most of the time to fit my needs but would sometimes reach it's limits with very low frequency effects which are harder on a sub to reproduce. If that's going to be in the mix I'd recommend going with the 15", two 12" Rythmiks or 1 with the intent of adding another when the budget permits.