Subwoofer budget max $700: new vs used, brand?

I need some input regarding a subwoofer purchase. I have a budget ceiling of $700 for a powered sub. My system's front end is digital, and I use a modest tube amp with Legacy Monitors. The system is used in a difficult room-vault ceilings, asymetrical wall placement, etc. I'm looking for something musical, not earthquake replicating. My musical tastes are eclectic: pop, rock, jazz, rap, folk. Aesthetics are probably not a concern as the speaker will most likely be placed out of sight.
Should I go new vs used for the best value?
Does a "self equalizing" speaker offer that large an improvement over a speaker integrated by ear?
Brand and model suggestions?
Thoughts, please.