Quad ESL 57 with subwoofer

Has anyone tried adding a sub to a pair of Quad ESLs?
If so, did it work out? What sub did you use and how did you connect it? Thanks.
I use subwoofers with both my ESL 63s and 57s.
I use a Dahlquist crossover set to 70 Hertz for the subs and the ESLs play full range.
Everything I have ever read about Quads says to limit the sub's hi frequency response to 60-100HZ.

FYI: Subs are DIY in 21 cu. ft. enclosures. Built from a design in Audio Magazine back in the 70s.
Have Altec 15" woofers. 416-8Bs I think. -3db @ 20hz.
I had a DIY sub with quad 57s with passive x-over but just between you and me the bass quality was better without the sub after removing the metal grills.
Thanks Dynalead. So are you running them from the same amp in parallel with the ESLs? I was thinking of powered sub(s) using a high impedance input from the speaker terminals...
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The best sub for any dipole is an OB. In the 70's a company named Gradient made one for the ESL63, with two 12 inch drivers facing in opposite directions in an OB "H-frame". A company named GR Research now offers a similar design, but with the addition of the Rythmik Servo-Feedback system. It is available only as a DIY kit, which is two 12 inch drivers and one 370 watt Servo Amplifier. You build (or have built, from the plans on the GR site) an H-frame into which the drivers are mounted. Serious, world-class bass the blends really well with dipole speakers. A pair will cost you about $1500 plus the cost of the H-frames.
If you can find a Janis 1W that setup works great with Quads. It has an outboard box/crossover, so what goes to the power amp is limited to whatever the crossover is. Puts less strain on the panels - a very good thing.

I've had this setup, it's the only sub I've heard that I liked with Quads.
I got a REL Strata 3 and it works well. I adjust it depending on recordings but for the most part keep it subtle. Definitely adds something to the Quads. I hope to buy a second one.

UPDATE! In the intervening years since my 2014 post above, I have learned that hardcore panel speaker enthusiasts recommend employing Magneplanar Tympani-IV bass panels as woofers, or even subs. There is one guy over on the Planar Speaker Asylum who uses them as woofers for his Martin Logan ESL speakers, with an Eminent Technology TRW-17 Rotary Woofer for the VERY low bass. That I would love to hear! 
I had the Quad ESL 63s and the Gradient dipole sub made for them.
To this day the best panel/dynamic speaker blend I've ever heard.  Not perfect, but far closer than I've heard since.