Best inexpensive sub for Maggie MMG's ?

new or used
I had good luck with Hsu subs when I owned MMG's. I just saw one for sale on here for $250ish, it appears to have been sold. REL is also a good choice, but a little more expensive.
IME, the issue w/MMGs and subs is optimizing the integration of the subs and the panels. It's tricky and IMHO depends on the getting x-over done properly as much as choosing the particular sub.

If you have an Audyssey (or equivalent) equipped pre-pro, you're golden, and you have a wide array of subwoofer choices that would, I suspect, work pretty well. If you do not, you might look at the Velodyne DD subs, which include their own integration software. It's a PITA (because the auto set-up sucks and it consequently requires endless manual tweaking for best results), but...if you're willing to spend the time to get it right... it works well.

Good Luck

I've heard smaller REL subs sound very good with mmgs.
((I suspect, work pretty well.))
((I've heard))
((when I owned MMG's))
Try two sealed 8 inch woofers
Adjust to preference

Not sure what you intended to say, but ...

I'll stand by my main point - precise level matching and smooth FR around the x-over frequency are critical to matching panels (or other speakers, for that matter) with subs and - IME - that's most easily achieved with Digital Room Correction software. I understand that some will disagree, but my post reflects my own experience.

As to the "I suspect", that qualifier explained the lack of a specific recommendation... kinda like "try two sealed 8" woofers". I know that you're a dealer (I was a customer when Hart owned the store and I bought a pair Cary 805s from you), but dismissing the previous comments as speculative and offering nothing more on point to the OP isn't particularly helpful...or gracious.

Parts express Titanic 10" sealed sub is what I have had for 10 years fed through line level out from my preamp. Blends very well with my MMGs. $400 kit or $580 assembled. The kit is easy.
VERTEK of Grass Valley,Ca makes unique subs for every combo on earth.He built one for me for my MAGNEPAN TYMPANI using two eights and a 250 watt rms mosfet amp that is just amazing in being so seemless.