SET amplifier recommendation

Just out curiosity, I plugged the ACA into my 89db efficient Sound Lab electrostatics (the 805 is 88db), and my Vision Acoustics Soloist (air suspension 88db), and yes it will drive the B&Ws, and probably quite well.

No doubt! But its not enough power on speakers like that unless the intention is to have the music mostly in background or serious listening at lower levels.

@jdl57  6 watts isn't going to do it on the 805. Quite simply you need some power.

That’s an interesting point, Ralph, which I presume derives mainly from the bandwidth limitations imposed by output transformers when they are used in SET configurations.

Would the bandwidth limitations you refer to apply to the small signal bandwidth as well as to the bandwidth at or near full power? Or just to the latter?

The issue of bandwidth becoming more limited as the total output power is increased happens with any amplifier with an output transformer. In SETs this issue is exacerbated. To give you an example, a Harmon Kardon Citation 2 which makes 60 watts has bandwidth at full power from 10Hz or so to well past 60KHz; a 300b SET which makes only 7 watts can't go as low or as high. This is why lower powered SETs like the 2A3 and the type 45 have gained ascendancy-  they have more bandwidth. But speakers with the efficiency needed to bring out their qualities are considerably more limited- you need well over 103 dB in most rooms to make amps like that work. Even with a 300B I would not be using a speaker of less than 99dB for the simple reason that if you use more than about 20% of the total amplifier power the distortion becomes unacceptable (that increased distortion is responsible for that 'dynamic' quaility for which SETs are known).

How different is the sound between 300b and 805 (LM 508AI, etc)?  

With SETs, the bigger the amp the less bandwidth. 300b is about as powerful as an SET can get and still be considered 'hifi' as far as bandwidth goes.

The B&W speakers are voiced for SS amplification and are never going to sound their best with tube amplification; possibly with the exception of a pair of Atma-Sphere OTL monoblocks.

We’ve got several customers that are very happy with this combination.

Our amps have a lot in common with SETs- quite literally, they are two SETs wired together differentially, with the output bridged. Triode, class A, with a character like SETs of distortion falling to unmeasurable as power is decreased. Unlike SETs they have about 1/10th the distortion, much higher power and wider bandwidth. The cost of replacement for a full set of power tubes for a pair of M-60s is similar to one good 300b.

The speaker is a problem for almost any SET.

But you've not exhausted all your options. You could also go OTL. Its possible to have many of the advantages of SETs (magical inner detail, amazing midrange) without some of the disadvantages (low power, limited bandwidth) in an OTL. I know people that run that speaker with OTLs, so I know it can work.