And what you would have also heard was a more defined and extended treble because those quads and these (ML's Acustats and other els's) go below 1ohm in the treble and start that impedance dip at 10khz.
Tubes I aggree with these speakers still sound good but they are definately subdued in the treble compared to a GOOD s/s amp that is BJT output and that can do current into low impedances. Like you found with the ML2 which can almost keep doubling it's current down to 1ohm for each halving of impedance.
This is not entirely true. Quads and Soundlabs have a higher impedance at 20KHz. In the case of the ESL57 and ESL63, the 20KHz impedance is high enough that even our small M-60 (60-watt OTL) can drive the speaker easily with no loss of HF performance.
In the case of ML, later Quads and Sanders ESLs, because the manufacturer is wanting to open up their market to more transistor amps, the impedance is quite low at 20KHz (some MLs are only 0.5 ohm).
In cases like these, you do as Tomcy6 mentions- you get a set of ZEROs (http://www.zeroimpedance .com) which allow you to easily drive ML or whatever! We have several customers doing just that with our older MA-1s (12-tube version) that only make 100 watts/channel, and they have no problem whatsoever making high frequencies on the speaker, with proper response to 20KHz and beyond.