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Here's an outside the box thought...

Posted by Lew on April 6, 2017 at 07:31:25:
If you are wondering what speakers to use, then you might consider buying a pair of Sound Labs, any of their full-range ESLs that use two audio step-up transformers, one for bass and one for treble, from the M1/A1 on up. Then remove the components of the passive crossover that divides the frequencies between bass and treble, particularly the RC network. Then substitute the toroidal treble transformer with either a toroidal or EI type full-range ESL step up with a 1:75 or 1:90 step-up ratio. Best quality you can find. This will do two wonderful things for an OTL amplifier: (1) The impedance of the speaker goes up to around 20-25 ohms across the spectrum, only falling off at the high end, and (2) Efficiency is markedly improved, over and above what one might expect just due to the impedance lift, because now you've removed that R in the RC network which was wasting amplifier energy. You can leave the inductor that rolls off the bass transformer in place, or not. It doesn't make much difference because the SL bass transformer rolls itself off anyway, above 1 kHz.
After I had done these mods to my SL 845PXs and during my first listening session, there were literally tears in my eyes; finally I had gotten the best out of the SL speaker. Three years or so later, I am still very very happy.
As to Quad 57s, you might consult Dave Slagle (Intact Audio) on how to get the most out of them, especially if you want to use 3 pair. It's been done by a guy here in the Maryland area under Dave's guidance, and he is very happy too.