Finding subs that integrate with electrostatics is notoriously difficult.Most sound too boomy and slow.I have made up one that uses an RCF L15/554k 15 inch in the RCF recommended ported box.A friend is using this with his ER Audio stats which are superb but can sound a bit thin.He is using a standard Chinese sub amp to drive this [150 watts].As these are really PA speakers and 100db sensitivity some e.q. is needed.The sub amp has a 6db boost at 35 hz which works well.The improvement is staggering-the sonic picture is now complete.
This sub amp also works well with my vintage Tosshiba SS30 speakers [the best dynamic speakers I have heard],and yet all other subs I have tried with these just can't keep up with their speed and dynamics.
I'm guessing why these RCF woofers work so well is probably due to their sensitivity,restricted excursion and cloth surrounds[always better than rubber]-although they are an expensive and highly regarded model.

JT