Hmmm. I posted a response, and I think I even went back and saw it up here, but is no here now.
So, lemme try again.
The '57 beams pretty seriously in the treble region, and the tweeter panel has an absorbent pad behind it so the radiation is monopole instead of dipole. The result is a relative lack of reverberant field energy in the treble region. So there may be a little bit of room for improvement there.
The tweak is this: Use a cheap 1" soft dome tweeter to fill in some of the missing treble energy in the reverberant field. We want its output to arrive after the first-arrival sound, which I define as the interval before the precedence effect or Haas effect kicks in. This corresponds to a path length difference of about 9 inches, so to be safe we want the tweeter about a foot or so behind the panel, sitting on the floor, facing up.
The tweeter is wired in parallel with the electrostat, and a capacitor is wired in series with the tweeter to act as a low pass filter. I suggest about a 2 microfarad cap as a good starting point for a typical 88 dB ballpark 8-ohm tweeter. Probably a good idea to avoid a 4-ohm tweeter in this application, as even with a series capacitor that might make the load a bit nasty for some amps.
I don't think it's necessary, and maybe not even desirable, to spend a lot of money on the reverb-field tweeter.
edit: Seems to be up there this time. Maybe I just previewed it and never really posted it before, only thought I had.
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