Something like 80% of our MA-2 amplifier production has been for Sound Lab owners.
The Quad ESl 63 as been a good match for our M-60 and MA-1 amplifiers. Martin-Logans (which have a lower impedance) usually require a set of ZEROs (http://www.zeroimpedance.com) to work with many tube amps. The CLS-1 is an exception- its impedance was set higher and so is easier for tubes (M-L really wants their stuff to work with transistors, so even though they sound better with tubes, their impedance is usually set lower so transistors will have an easier time with them).
Some of the old Accoustats were an easy load for tubes and some were not, as Accoustat was trying to crack the solid state market too. If one of the higher impedance Accoustats, IME 50-60 tube watts is plenty of power.
The fact of the matter though is that the nature of tubes where they do not always act like a voltage source (which is the ability to double power as impedance is cut in half) is an advantage when trying to drive the higher impedances that an ESL often has. Unlike a box speaker where the high impedance in the bass can be the result of a box resonance, the high impedance of an ESL has nothing to do with resonance and so they still need power to drive them even though the impedance is high. Tube amps do this better than transistors.
As an example, a 600-watt transistor amp can't make any more power into a set of Sound Labs than you might get from a 150 watt tube amp. Put another way, a lower powered tube amp can often make as much or more power than a more powerful transistor amp when driving ESLs.
In short, ESLs and tubes have been one of the better combinations in audio for the last 55 years.
