Subwoofer Options - REL, BagEnd, JMLab Electra Sub

Although this is not on your list, a Vandersteen 2wq should also be considered. I have MG 3.6R with JM Lab Electra SR30 for surrounds. I use the xover in my Theta CasaNova. For me, this sub blends perfectly and just extends the bottom end.

In this order:
ACI Titan II LE, Rel Storm (or maybe larger), Vandersteen 2WQ

I have to agree with those who advocate REL Subwoofers. I own a Storm III that I use with my B&W N803's. It integrates like no other subwoofer that I have used in my system when hooked up through the power amplifier via the Neutrik connection. Very musical. I would think that there are subwoofers that offer more boom for home theatre, but no other subwoofer I have listened to under $5000 integrates as well or is as musical as the REL Storm III.

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.