These are intended for a small room- 10 x 12 or something along those lines. I often listen at lower levels, ~85 dB peaks; but of course I can also listen at levels up to the point where the system shows strain.... so I think it all really depends on these panels. I do not have them yet. I have no experience with them, they are ER Audio (Australia) model 440 panels. Once I get them and do some testing I'll have a better idea of their capability in terms of SPL and how low in frequency they actually work. I've read that they can be crossed over at 200 Hz with just a capacitor (which would be a first order filter) but I'll have to measure them before I accept that.
I should mention that I have a lot of very good speaker systems (rebuilt ESL-57's, DEQX triamped Magneplanar MG 3.6's, etc) and so these are more of a the-fun-is-in-the-building project rather than a these-will-be-my-main-speakers project. I always wanted to make an ESL hybrid and I've been intrigued with the GR open baffle servo woofers so this looked like a chance to try both.
After reading the advice here I have a picture of a panel-plus-single-12" idea, since everyone seems to be saying that the output of the 8" driver is not very high. You know I just can't imagine that a pair of these small panels is going to be able to produce 100 dB SPL plus levels at 200~400 Hz even in a small room. I've read they have a larger-than-usual excursion for an ESL, so maybe they can. But it all remains to be seen. Or, rather, heard.
Beside the extra output a pair of woofers (either 8" or 12") per side provides over a single, there is another benefit to be gained by using a pair---employing them mounted in the opposite-facing/opposite-polarity dipole fashion: The OB/Dipole sub will then have the same figure-of-8 radiation pattern and room-loading characteristics as the ESL. Yes, the wavelengths of low frequencies are so long as to make bass omnidirectional, but a dipole sub is still better than non-dipole for use with a dipole speaker because: 1- The sub has a null at either side of the H- or W-Frame (a result of the opposite-polarity waves from the front and backing meeting at the sides, causing cancellation (+1 plus -1=0), as does the ESL. Those side-nulls allow the sub to be placed right next to either a side wall or the ESL itself (or both) without a sonic penalty. The sub can even be positioned laying down, for use as a base for the ESL if you so desire (with some form of isolation between the two, of course!); and 2- The drop-off in output of a dipole and a non-dipole differ from one another. When you set a balance between a non-dipole sub and a dipole main speaker, that balance is so at only one distance from the pair, as the output of the two changes with distance at a different rate relative to each other. With the dipole sub/dipole speaker pairing, it remains constant as listening distance changes. That's a big deal! The Finnish company Gradient made an OB/Dipole sub in the 80's specifically for the Quad ESL63, and it worked great for those reasons and others (decreasing the excursion of the ESL diaphragm for one, thus reducing distortion and increasing maximum SPL possible from the Quad). And that was without the Rythmik Direct Servo Feedback and the superior GR Research driver!
Once you have heard the OB/Dipole Sub with your little ESL panels, I'll bet you'll want to try them with your Quad 57's, with which they meld beautifully. I use a pair (each with two 12's and an A370 amp) with those Quads, as well as with a pair of magnetic planar speakers (though I have a pair of Magneplanar Tympani T-IV's, I haven't had the subs and them set-up together. I use the subs in place of the stock woofers in the Eminent Technology LFT-8b, crossed-over at 180Hz). Congratulations---you have discovered THE sub for use with any and all dipole/panel loudspeakers!
