Using tube amp with electrostatic speakers.

Most all tube amps use output transformers with impedance taps that work just fine into 4 or 6 or even 16 ohms!

The Zero is of NO VALUE to most tube amps!

Don, the impedance of many electrostatic speakers, including most Martin-Logan models, decreases to very low values at high frequencies. The SL3 model the OP referred to, while having a nominal impedance of 4 ohms, is spec’d at 1.5 ohms at 20 kHz. Some other M-L models have impedances at 20 kHz in the vicinity of 0.7 ohms.

Most tube amps have effective output impedances on their 4 ohm taps that are comparable to or even greater than 1.5 ohms. The interaction of that kind of output impedance with that kind of load impedance variation will result in a greatly under-emphasized upper treble region relative to the response that would occur with a solid state amp. A Zero would either eliminate or considerably reduce that problem (note Ralph’s reference to avoiding high end rolloff), and I suspect it would also reduce amplifier-generated distortion.

Regards,
-- Al

P.S. to my previous post: The Audio Research V70 that was referred to in the post by Lostbears had a MUCH lower effective output impedance than most tube amps, due in part to very liberal use of negative feedback. Specifically, according to arcdb.ws it was spec’d as utilizing 20 db of overall feedback, resulting in a damping factor of 30 (the highest damping factor I can recall ever seeing for a tube amp), which theoretically corresponds to an effective output impedance on the 4 ohm tap of 4/30 = 0.13 ohms, essentially in solid state territory.

Generally speaking, Audio Research and also McIntosh tube amps tend to have lower effective output impedances/higher damping factors than most other high quality tube amps.

Regards,
-- Al

Yes, I also agree with atmasphere as I once had some Wilson Sophia speakers, 4 ohms nominal impedance, and a pair of Fourier Panthere OTL amps whose output impedance was around an ohm or so.  Adding a pair of autoformers made a huge difference in sound, particularly dynamics, bass depth and tightness, soundstaging, detail, just as you would expect with the amp being able to more fully control the speaker drivers.  Once I did this I loved what tubes can do for panel speakers.

Ralph was too modest to claim it, but he is a high level expert on tube electronics and electrostatic speakers. I would trust him implicitly.

Furthermore, I have run a pair of his 'circlotron' tube amplifiers, Atma-Sphere M60's, into the ESL panels of ML speakers. They sounded very good indeed.

jimbreit - I have some experience with your question . I also posed the same question a few years ago . Only a few people responded with less than great advice . Most stated a couple hundred Audio Research watts would be needed with my stats (King Sound). I pretty much was convinced i would never have a pair of tube monoblocks in my system due to the economics involved and focused on ss . 
Fast forward to last year . I picked up a Primaluna Dialogue 2 integrated rated at 35 wattts . At the time i had 6 pairs of boxed speakers that the plan was to use the primaluna with . One day i decided to put it in the main system with my King Sound stats and was pretty impressed with what i was hearing. Zero distortion as the volume increased to very high db's . I was able to play the primaluna just as loud as the solid state .
I have never purchased a amplifier unheard but plan on getting a Primaluna Dialogue premium hp some day . I can only imagine having the extra 35 watts over the dialogue2 will sound like .