Magnepan 1.7 and Subwoofer help/recommendations

I seem to have gotten myself into a bind with purchasing a subwoofer. Hear me out. I recently acquired myself a pair of Magnepan 1.7s, and they sound great. The problem is at about 90hz, the speakers completely fall apart. Obviously, it's practically incapable of producing frequencies below 30hz, which is fine with me, but my problem is that at about 50-80hz, it's a got a big honking hump of bass that overpowers a lot of the details in the mids and highs. It's like they know that they're about to give out, so they give one last big push. I would honestly rather have less bass in that area, which is what leads into my subwoofer problem.

I use my Benchmark DAC1 USB as a preamp, using the XLR outs going to my Bryston 4B with only XLR/TRS inputs. After having read some suggestions on subs that match well with Maggies, I made a list. Practically the #1 suggestion I saw was Vandersteen 2wq. I saw one online yesterday used for a good price, and I called Vandersteen, and they said that in order for me to set it up, I would need to purchase the M5 crossover, which would cost quite a bit of money ($600 minimum), which I would rather not spend. On top of that, there is only 1 Vandersteen dealer that I know of in Canada, and they haven't called me back as to whether or not they have the M5.

After looking online at a bunch of options, I've gathered that all of them pretty much only have RCA ins and outs. So, my question is: Where can I find a subwoofer that will a)match well the Magnepans
b)I can hook given the limitations of my system
c)Under $800 used
d)Goes all the way down to 20hz (Kind of just a bonus, not really necessary)

Presumably someone here would know more about this. If you do, please help me out and offer some advice. Thanks in advance for any responses.