Using tube amp with electrostatic speakers.

I am fairly sure that the A1s would look like my 845PX with respect to impedance, which is "very high" up to 100Hz (50 to 100 ohms), then around 4-5 ohms up to around 1kHz to 2kHz where there is a sharp notch dip to 2 ohms or so, then back up to more reasonable values, then back down at 5kHz and above, to 2 ohms or less at 20kHz.  This is from memory.  (I could REALLY hear an improvement in vocals when I got rid of that 2-ohm dip; see below.)

You can probably assume, if you don't want to or cannot measure, that the B1S will present a fairly high impedance to the amplifier, and in that respect it should be an easy load for any tube amp, but the question would remain how much amplifier power you need to make it sing.  You might ask the folks at Sound Lab.  

One option you didn't mention but which works quite well is to bi-amplify your A1s.  I've never done it, but you can experiment with that idea.  See if you can find the old "Sound Lab Owners Group" archives for some hints.

And finally, you might do what I did with my 845PXs: (1) Acquire a full-range ESL step-up transformer with a turns ratio of 1:75 up to 1:90.  Plitron makes a nice one, toroidal at 1:75, but there are several others. I used a EI type transformer made in Australia that is unfortunately no longer available.  (2) Remove the existing treble transformer in your A1s and also remove the RC network that effects a high pass filter feeding the treble transformer. That RC network is the problem; it creates the low impedance dip right around the crossover point, because of the value of the R that SL chose to use. Also, the R (typically 5 to 8 ohms in value depending on how old your speakers are) soaks up amplifier power, because it is in parallel with the panel at all frequencies. The intrinsic impedance of the panel is mostly way higher than 5 to 8 ohms, so most amplifier energy flows through that R.
(3) Install the full-range transformer in place of the existing treble transformer AND the RC network, so there is no longer an RC at all.  You can retain the inductor that feeds the bass transformer.  Make sure the two transformers are "in phase".  And then your A1s can be driven by your 60W tube amplifier all day long. The resulting impedance is going to be 20-30 ohms from about 100Hz to 5kHz, going down from there but fairly constant between 100 Hz and 2kHz. And the efficiency is ridiculously improved.  I doubt my speakers draw much more than 20W, steady state.

Or finally, talk to Roger West about upgrading your treble transformer to his latest device, which is more nearly full range. That allows for a lower crossover point and also gets rid of that midrange dip in impedance, according to them. With that option you keep the crossover (but change the values according to his instructions). Plus, this mod is factory sanctioned, unlike what I did.

Does the 4 ohm tap in this case make sense to you?

@garyosoba Yes.

Lew, I've heard the new Sound Lab backpanels a number of times and its pretty obvious that Dr. West got it right with this one. Its much easier to drive (we can do with an MA-1 what took the MA-2s before) and sounds better too.

Ralph,
Your information about the new Sound Lab treble transformer ("back panels") is consistent with what I have heard too, from other Sound Lab owners.  They all seem to like it, and they all sense the improvement in sound compared to the old set-up. Even the one or two guys who first tried "my" approach and who were not satisfied, are now happier with the new toroidal treble transformer.  No doubt that is the simplest thing to do, if one owns an older set of panels.  

Note to others that getting rid of the crossover entirely was not originally my idea; it was communicated to me by "Will" in Australia, via SLOG.  Will also helped me and others to obtain the Australian-made full-range 1:90 step-up that he used too to modify his 945PXs. I bought the last pair of the original run of those transformers, but the vendor had more of them made due to the demand from among SL owners. There may have been something odd or different about that second batch of production units, because I know of at least two guys who were not happy with them. We'll never know why, with certainty.  This is why I would recommend the Plitron 1:75 full range toroid, not the Australian transformer, to anyone who wants to try the mod I describe above.  I personally have never thought twice about adopting the new SL toroids to replace what I've got.  Anyone in the DC area is welcome to stop by my home for a listen.  

To be more clear, the Plitron toroid is a full-range step-up that one might choose to install if one wants to go with no treble crossover at all.  SL sells a toroid that requires a high pass filter in front of it; it's not designed to run full range.  But with the SL toroid, the hi-pass crossover point is down around 200Hz, which does dramatically improve the impedance curve compared to original and the efficiency of the speaker.

Lewm,

Yes, I did ask many questions with Dr. West who was more than gracious with his time and help.  He told me that a 300W SS amp was more than enough to drive the B1s.  I am currently using a Crown K2 (350W @ 8ohms) on the B1s.

I am trying to absorb and understand your modification.  My speakers did not work initially so I had new mylar installed by Soundlab on the A1s and the B1s.  My panels had the Toroid l in it.  A few years ago, I found someone selling upgrade parts that they did not install in their system which I purchased.  The upgrade parts were a large 36uf capacitor (pio) and the Toroid ll transformer (the latest version).  I consulted Dr. West who confirmed the parts.  I removed the brilliance control and replaced it with a four ohm resistor (which was about the setting I had it at).  Dr. West told me I could remove the inductor coupled to ground in the circuit but I have a 12 or 14 ohm low inductance resistor to ground.  I believe the resistor was larger than what was in the circuit but Dr. West said that the Toroid ll can extend lower in frequency and that it would be fine.

A couple things mentioned by Dr. West.  He said the B1S can be used as a full range speaker which I found interesting.  He said that when they used the B1S with the A1 panels they would crossover at 500hz.  Running the A1 at 500hz would essentially be operating the A1 with only the toroid ll.  I tried it but I couldn't get it to integrate correctly (I will likely try it again in the future). It sounded better at an 80hz or less crossover.

Once again, thank you for your knowledge and help.  It sounds that I can drive the A1 with the tube amp with the 2 or 4 ohm tap.  I question whether I will have the voltage swing with a 60w amp for the midbass if the impedance gets high from 100hz to 1khz.

Best Regards,
Gary

One other thing.  Have you ever tried the zero transformer?  It sounds like it would help especially with low power amps that could have trouble driving the difficult load.  The only thing that could be at issue is the lower frequencies that get multiplied up to 40-200 ohms.  I don't know if this would be an issue for some amps.