Quad ESL 57 with subwoofer

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.

All you need are,8-10" subs, a pair is ideal. Drive them from your main amp using the high level inputs, so not run cables from your preamp, set the crossover at 40Hz, adjusting the volume to blend, use a piano and a vocal recordings, when either becomes to thick/heavy sounding you raised the volume to high, after you are happy try 50Hz, you may have to adjust the volume  a bit, choice is yours on what you prefer, don't crossover higher than 50Hz.