What amp do you use with your QUAD'S?

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.