what type of subwoofer is good for ribbons

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.

Like Racamuti, I also run a pair of TBI subs in stereo. They integrate perfectly with Quad 989s. Highly recommended. Robert

I also will throw the TBI into the ring;I am looking at them for my soundlab m2's as well.

Duke (Audiokinesis) is too honorable to blow his own horn. However, nothing prevents me from doing it for him. His SWARM system is outstanding, if:

1) it's in your price range
2) you have the space to place 4 small-footprint subs
3) you can deal with the cabling for #2

I own his Planetarium Betas, of which the SWARM is an integral part. If, however, I were an owner of Maggies, Quads, or the notoriously bassy-shy M-L 'stat, I'd be all over these.