What amp do you use with your QUAD'S?

Trebleclef,
I used to own ESL63USAs and tried them with Cary Rocket 88, and then later with a Berning ZH270. The Berning was the best I've heard with the Quads. Much better than the Cary in most areas of performance. More power than you need. Exceptional clarity across the mid-band.
If you move to a different speaker later, as I did, the Berning gives you lots of options down the road. It is quite exceptional IMHO.
Cheers,
Spencer

Well, I may have to save for one...:-)

Harhau - My take on the 400a being used for driving stats is to do away with the "stacked" output devices and make a bank of simple emitter followers, using however many of the devices you like. Also, from it's output design and how it's integrated with the active bias circuit, it has a rather high open loop output impedance and could probably be lowered for driving stats. Of course, this entails somewhat of a redesign, so would leave that to you. I know of at least one person having used it to drive some acoustats and blowing fuses was a problem when pushed. Since you mentioned you were bringing a 400a back to life, just thought I'd chime in....:)

I used then first in the 60s and was a dealer for years. Most of the amps I used are long gone but a friend of mine who has a pair I sold him in the 70s formerly used an Audio Research but now uses a rebuilt tube amp from the golden age,I have forgotten which. A rebuilt Dyna, Scott, Fisher etc would work well.

Harhau - I just rebuilt a 400A to use with my Martin Logan CLS's and it drives them pretty well, doesn't seem strained or anything. But, it is by no means an ear shattering experience. It's a work in progress so it just keeps getting better. The only reason I mentioned awhile ago about changing the output stage was to gain more voltage swing which stats need. The quads may not need them. I even seem to recall that the 57's limited the voltage after a certain level, 40 volts comes to mind. The 400a is indeed a different beast and a kick to work on, having enjoyed myself immensely and even threw in some circuit mods that booted it all up a notch. Hotrodding a power amp is not hard to do since they never seem to leave the manufacturing line as well made as they can be. But it does seem to be a stable amp.