Sealed subwoofer for ESL-63

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.

I have Quad 2805s, and added a B&W PV1d because it was claimed to be very fast and easy to integrate with the stats. The result was disappointing, even in my large listening room. The combination sounded boomy unless I reduced the level to a point where I could not quite hear the sub anymore. All this was largely solved when I added an Antimode 8033 room equalizer for the sub. The bass now integrates perfectly with the stats.
However, Duke is also right that multiple subs are another way to address this, and over a wider area (his Swarm system seems a very good way to do this), so I will buy at least a second PV1d (not sure I can persuade my wife to live with more than that - and quite rightly so), still combined with the Antimode (and one sub with inverted phase).