I can also recommend the RELs. I have two Stentor IIIs and am amazed at how well they seamlessly integrate (I have them crossed over at only 23hz [probably lower that you would because my main speakers are full range] so they "come up" under my mains and do not impose on them). In addition, they have both extended my bass response and tightened it up as well.
I should point out though that set-up is crucial and they are not their best when used like many typical subwoofers.
I should point out though that set-up is crucial and they are not their best when used like many typical subwoofers.