Besides the REL, if you want a sub that uses a high pass crossver (so that your speakers and amp don't handle bass below, for example, 85 hz) consider a Velodyne servo sub like the FSX 12 which can be had used for around $600.
When I compared subs, some less expensive models (including Velodynes) weren't bad, but to me they still muddied the sound somewhat.
When I compared subs, some less expensive models (including Velodynes) weren't bad, but to me they still muddied the sound somewhat.