Subwoofer Options - REL, BagEnd, JMLab Electra Sub

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 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.

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

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.

You can't go wrong with a REL. At least the Storm III. The Stratus III is a nice sub, but does not go as low.

I can't say which of the above will mate the best with your JMLab's, however, I purchased a REL Storm III (used for $1100, delivered) to mate with my Maggie 3.6R's. It works beautifully. The REL has wonderful musicality and does not, and never has, drawn attention to itself. Hook it up, and boom, you have real extension in the bottom! I drive my maggies full range, so the sub just supplements the bottom (probably the same thing you'll be doing). I have only heard 1 sub that used an active xover that didn't damage the sound (that was the subs that come with the Pipedreams).
My advise (for what it's worth) is that there's no xover, like NO XOVER!
Go to audioreview.com and read some more comments on the REL Storm III sub there. I think for the money you simply can't beat it. I am very happy with mine.
Good Luck!

I'm in the same boat. Looking for a sub with the Mini's. I am almost inclinded to go with the Electra since it should match well with the JM line. However the REL Storm III sounds interesting.