I too am running an ESL system: 2 Quad 2905's and 2 Quad 2805's perpendicular to the 29's (all modified). I am running an isobaric sub with a 50 Hz cutoff, but frankly it doesn't do very much. Almost all of the bass augmentation comes from Magnepan DWM bass panels, powered by big Brystons (a 7BSST3 per DWM). The 7B's are potent, fast, and clean enough to drive the DWM's flawlessly - overkill, really.
The DWM bass panels use the latest push-pull technology favoured for the big panels. They are very fast - basically indistinguishable from ESL's at 500Hz, an octave above middle C. The stock crossover is at 200Hz, but it's easy to bypass that and do it right with an electronic crossover. The DWM's are fairly flat to 2 Kz and still produce a decent signal at 5KHz, so they are ideal for mid-to-upper bass augmentation in the critical region of 40-300 Hz.
I like them very much. YMMV