Please don't be cross with Thierry. It is not unusual for people to ask about obtaining a theremin that sounds just like Clara Rockmore's, and we sometimes need to explain that it's not going to make them sound like Clara Rockmore.
I'm guessing, but if I posted on a violin forum that I was quite new to the violin, and wanted a violin just like the one Heifetz plays I would not be surprised if someone told me the same thing.
Also, "what is a good professional theremin?" is a bit of a tough question at the moment - there is something of a hole in the market since Moog dropped the etherwave pro. Here we differ from the violin world, where - I imagine - there is a good range of quality instruments available, to suit most preferences and budgets.
90% of the history of the development of the theremin electronics can be attributed to precisely two men, Lev Termen and Bob Moog, both of whom are no longer with us. Bob Moog spent a lot of years designing theremins until he got to the ethervox, arguably his best instrument, and probably the least commercially viable theremin ever marketed, although the RCA Victor is a strong contender too - because the scarcity of excellent theremin makers is echoed in the number of professional classical thereminists.
Moog Music's current focus appears to be on the electronic music market, where the etherwave ticks a lot of the requirements boxes for amateurs and professionals alike, and is the de facto recommendation for a newcomer with the budget for one, or as a next theremin for someone making a more financially cautious entry into the world of thereminists.
I should also add that we get a very broad range of people interested in the theremin, from skilled professional musicians to people with no musical experience whatsoever, from the purest classical to the most extreme experimental, (another common question is "how do I build a theremin into my guitar" which might be likened to "if I drill out the neck of my violin can I use it as a flute?") and performance and installation artists too, as well as electronics geniuses and kids wanting something quick and easy to build for a science fair. And we all have different requirements and opinions. Sometimes it is hard to know what is the right answer to a simple question to best suit a particular person.
And of course, we're all a little bit... er, "insane" is a strong word, but it is an eccentric instrument, there's no denying it, and most people react to it as they would react to naked skydiving: interesting to watch, but they wouldn't want to do it.