Just auditioned Martin Logan ESL vs 60xt

 but I've never heard an amp just sound awful then sound great on a pair of normal speakers.

That's because it couldn't handle the ESL's loading.
You needed to hear the ESL's again with an amp that can take 1ohm loading and do it comfortably without rolling off or becoming unstable, then you would have been very impressed with the highs and their extension.

Cheers George 

...speaking of amps for electrostats: is there a danger in using inappropriate (i.e. unable to cope with the <1ohm load in the high frequencies) amplifiers with such speakers? I have a pair of Martin Logans (the Classic 9) and I was thinking of trying some other amplifiers with them. Do I risk a nasty surprise if the amp gets unstable or whatever (apart form less than great sound)?

As for the topic of this thread: in my (limited) experience, at least as far as my speakers are concerned, they seem to roll off slightly starting from ~ 9kHz in my quite absorbing room, but the amount of treble (not only the last octave) depends a lot also on the room acoustics. Less absorption and you get plenty of treble (perhaps even too much lower treble in my setup).

A lot of it might have to do with setup. The panels radiate front and back. They rely heavily on ambience from the area behind the speaker and proper spacing. They're also very directional despite the curved panel. If they sound dull that might be why.

You can try different amps with them. I'd expect driven into a low load impedance some amps will either go into protect mode or start to sound constrained or compressed. There are some marginal designs out there that could die but I'd think you'd know it wasn't happy before that.

I don't think you could kill a tube amp with one. There are a lot of good amps for not too much money that will run them all day. 

Martin Logan recommends amplifiers that double in power each time you go down in ohms (all the way to 2ohms or better. Example: 120w/ch @8ohms, 240w/ch@4ohms, 480w/ch @2ohms. The difference in the sound with this in mind is jaw dropping. I have gotten the ESLs to sound good with a Class D NuPrime STA-9 though ($659). I run a tube amp and it just didn't give the instruments realism. I bought a Krell amp used and serviced, and my world went into a real sound system that gave a sound that was one of those "regardless of price" quality. I have since bought more powerful Krell amps and upgraded to a much more expensive ML Montis that I bought used. Also with (2) REL S3 subs sitting on the inside of the Montis, I have dialed the system into a finished state of the art sound system. Martin Logan base models ESLs need some sub love as well. Going up to the Anthem Correction ML models will give you the proper bass in medium sized rooms. But don't underestimate the magic of the REL sub systems. Don't bother with electrostatics if you can't give them proper amplification. I learned this thru experience. Don't be afraid to buy some items used for your sound system. Most of us..... it is the only way to afford "high end" sound. My 2 cents.

I stumbled on this older thread helping a friend to do a proper evaluation.

It’s great folks here truly recognize you cannot just walk in to a BestBuy Magnolia type store and hear what the Motion or ESL Martin Logan speakers can do with a lame HT receiver through a junk speaker switcher with layers of poor interconnects daisy changed with garbage speaker cabling, bad connectors, and limited source components and expect it to perform correctly. Garbage in, Garbage out, and weak links end-to-end for sure.

They do the best they can with rushed A/B comparisons and lots of surrounding noises going on in a retail chain store. Pull the same speakers out and put them in a worthy setup at home. Much different.

Take the same Motion 40XT, 60XT, and ESLs, hook them up at home with a high quality dedicated 2-ch amplifier (solid state or really good tube system) with some real transformers and nice tubes behind it and experience a very different type of result and musicality from the exact same speakers. There are some very good speakers out there that sound much better in a worthy system at home.