what type of subwoofer is good for ribbons

Since the only Maggies w/true ribbon drivers are the 3-series and the 20-series, you will be in a very good class of panel.
However, I'd skip the sub until you decide you really need it. The 3.6, soon to be 3.7 is pretty good while the 20.1 is very good. (for bass output).

Match a sub w/ panels? Cross over very low...near the lowest rated frequency of the panel. I cross over my 1.6s (not ribbon) at about 40hz or less.

setup is key. Also, for Maggies, you'll need a good amplifier. NOT because they are a bad load, but because of fairly low sensitivity. Make sure you have a large enough room to integrate larger panels and can place them into the room far enough....3 or 4 feet from 'front' wall, as a start.

My HSU Research sub has 2x ports and a single optional plug. I use the plug and run in 'maximum extension' mode. Good output is reached down to a usable 16hz...the lowest organ pedal note. Bass guitar, standup bass and kettle drums are all musical. The lowest octave of an Imperial Grand is real, in room.

Go audition the Maggies without sub and see if you can live without it. I'll bet that the larger panels can mostly do without.

Asymmetrical sub placement is good.....Very Good. I read the Harman white paper some time ago and that was one major conclusion.

However, not having deep enough pockets to afford even a pair of the modest sub I ended up with, I must rely on my wacky Asymmetrical ROOM.
8 sides, 2 of which are at 45degrees to the rest, while the ceiling is about 11'6" at the off center peak.
This room really helps and acts to break up bass modes which may otherwise be troublesome.

http://www.harman.com/EN-US/OurCompany/Technologyleadership/Documents/White%20Papers/multsubs.pdf

I don't know how I found it.......and can't read it. My eyes are watering so bad from this cold I can hardly see.

Somebody read it and summarize, please.
Also, please note in your answer how well your listening space conforms to the test space....mine? It couldn't.....I have an 8 sided asymmetric room with a vaulted ceiling.....a largish LR / DR in the California fashion.