Looking for a Sub (under $500) that would mostly be used for Home Theater room (750 st ft).
Looked at Martin Logan Dynamo...
Any other ideas?

Looking for a Sub (under $500) that would mostly be used for Home Theater room (750 st ft). Any other ideas?

What sized room? I would recommend e-mailing the following companies and asking their advice.

www.svsound.com http://www.svsound.com/products-sub-box-isd.cfm

www.hsuresearch.com http://www.hsuresearch.com/subwoofers.html

www.********** http://www.**********/products_category.php?section=subwoofers