I think I have been lucky and not inreadably critical, off axis the reproduction is very wrong for the electrostatic, while the monopole operating at such low frequancies has no real effect on or off axis. When using one subwoofer performance was poor, but having two seems to work well enough for me, maybe its not perfect but its good enough.
I have not done enough real tests as making each box takes far too much work for me to try again soon as I have other HiFi projects to build that are far more important than low frequancies from my speakers, that you dont need in most music.
This said I used some 7" Linn Helix moving coil speakers in a slot loaded bass reflex before I started my subwoofer project and the sound was not right, and I could not seem to make it right. I tried a few cross over solutions and none worked, but did conclude that 100-110Hz would be a good place to cut off the ESL-63 Electrostatics I have. Later I measured the frequancy responce of the Linn Helix speakers and it was nearly as lumpy as the Quad ESL-63 in the bass. The sealed units I built still have some resonance but it is but not nearly as bad.
Quad ESL-63 speakers have enough bass for me without a subwoofer under most situations, so the next iteration of the preamp will have a switch, allowing the preamp to sent the output to many different amplifiers, I will make it so you can select to send signal to the active cross over with various combinations, so I can use the subwoofer + electrosatatics, use the elctrostatics without the subwoofer with no filtering out of bass, and for late at night use the electrostatics with a high pass, and possibly anouther output to drive a headphone amplifier, all with an easy switch as I am happy with the bass I get from the subwoofer and now care about the usability. I am not convinced my neighbours will be as pleased witgh the low bass quality late at night, so currently I just switch off the subwoofer amplifier.
By the way a great advantage of dipole bass, (as produced by the ESL 63 as I have no other dipoles that go low) is that they loose bass dramatically off axis. This means that the bass does not sound anywhere near as loud as you would expect outside the room the speakers are playing in.
Regards
Owen