I would look at those for a start:
http://www.hsuresearch.com/products/stf-2.html
http://www.outlawaudio.com/products/lfmcompact.html
http://www.edesignaudio.com/product_info.php?cPath=2_41&products_id=406
http://www.edesignaudio.com/product_info.php?cPath=2_41&products_id=892
Although, I am not sure that in this price range you'll find a sub that will offer much more that the M80s unless of course you run them a few dB hot and at the volume that you seem to be listening, I am not sure they can even keep up.

Bruno
M80s/VP180/QS8s/EP600/AVR-890
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"The problem is choice..."