Sealed subwoofer for ESL-63

I've had great luck integrating a REL Stadium with Quad 63's and later pair of REL Stentors with Quad 2905's. I had Jim Smith help me set up the 2905's and the results were seamless. I would highly recommend REL subwoofers, ideally a matched pair, placed behind your panels if you can put the Quads well into the room.

according to stereophile's measurements for your speakers drop like a rock at 80Hz which makes them excellent candidates for subwoofer integration.  
the key ingredients for perfect sub integration are as follows-
1) subwoofer with polarity invert switch
2) subwoofer with continuous phase angle adjustment 
3) subwoofer with high level (speaker line) inputs
4) subwoofer with flat usable frequency response.  
Some REL subs and JL Audio subs have these features / attributes which is why they blend seamlessly with a little work.
I have the JL Audio D110 integrated perfectly with Harbeth C7s and can recommend it for you application.
*inverted polarity switch helps to eliminate peaks in the response through summing.  
*continuous phase angle allows fine tuning to completely eliminate any peaks or cancellations by aligning the pulse with the main woofer at the crossover point.  
*high level inputs allow better blending because your sub sees the exact same signal your main speakers see.  run in parallel to your amps connections- it creates a benign high impedance load to the amplifier. 
*a flat useable response is what makes a sub sound musical- being able to play many bass notes and not be a one note wonder.  
The JL D110 is an excellent sub at it's price point.  

It’s all about getting the two different masses to behave as close as possible at the xover point.

For a sub to react as fast as an esl-63 at the xover point, it needs to have a reactance speed as close as possible to the speed of the 63’s at 80hz which has virtually no mass, hence the smaller lighter diaphragm/s sub driver/s will be closer, and to help them even more, a servo feedback will help again.

Cheers George

+1 avanti1960

Buy a Martin Logan Sub. After all they build Electrostatic speaker. I use one Depthi-i sub with excellent results with Crossover at 35. Seamlessly blends in and makes my 63 sound even better but without hurting the mid range purity.