Subwoofer Options - REL, BagEnd, JMLab Electra Sub

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.

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!

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.

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.