Subwoofers for Stacked Quad ESL 57's

I experimented years ago using a single pair of the old esl, decca ribbons, and could not get a conventional woofer to blend well. There is a possibility that you could use a planar type woofer- years ago, HP had Tympani bass panels with Quads, I think. One designer i talked to more recently said he modified a Quad 63 to act as a bass driver to augment a pair of 63's. One of my long range plans is to restore my old pair ( I have both the 57 and the 63 Crosby mod) buy a second pair of 57's and do a vintage stacked Quad system. So, I'm interested in what others recommend here.
A horn loaded woofer might be interesting....
Do you have a frame for the Quads?
I have old literature on stacking Quads somewhere, along with diagrams.
Cool project.

Thanks very much. I do have the frames. I have heard that a transmission line sub is the way to go for the most seemless integration and quickness to keep up with the stacked quads, but I am very open to fellow Audiogoners opinions. I would be interested in the diagrams because I am considering building new stand as well.

Rmarcus - I sent you a PM, please let me know if you got it.
Best,