Thanks for all the suggestions. I have purchased an Innersound preamplifier, which is on its way to me now. I have never heard this unit, so I will be auditioning it carefully, and intend to post my findings. There is not a lot of info available on this still fairly new design, and it is apparently not the most widely distributed, so I couldn't try it in a showroom. Their website contains a link to the one review I have been able to find, by Frank Alles in The Stereo Times webzine (highly positive, FWIW).
From the info on Innersound's website, and much very detailed (and prompt) correspondence kindly directed my way by the proprietor, Roger Sanders, I was able to make the preliminary determination that this design probably would fill my brief as outlined above as well or better than the competition mentioned in the posted responses above.
Getting a newer model from my current manufacturer of choice, Conrad-Johnson, or looking into the BAT or Sonic Frontiers options, is still always a possibility if I'm not blown away by the Innersound, but I want to try and see if I can be happy with the sound of the system without the hassles I've had trying to find and maintain problem-free tubes for the high-gain, small-signal preamplification stage. The Innersound is an FET design that has all the features I wished for, and as far as I can tell, none of the tube options quite comes as close on the features front.
Innersound offers a very generous, no-risk, home audition for 30 days, and even offers to pick up the shipping charges both ways if a customer does not elect to keep the unit (I myself have taken a chance on a warranted dealer demo at a sharp price [thanks to A-Gon], so I won't enjoy this privilege). From what I've gathered, their emphasis is on complete transparency and neutrality combined with an unusually high degree of flexibility and control. (Incidentally, Roger says Innersound is getting close to introducing tubed versions of both the preamp and their ESL amplifier, as requested by their customers.) From the level of personal attention that I've experienced from this maker so far, I would encourage any curious or interested folks to contact Roger for information on any of their products - I'm truly impressed (even though I don't necessarily agree with all the positions he takes in his white paper). And for the record, no, I don't have any affiliation with Innersound, its dealers, suppliers, or distributors - I'm just a guy in search of his next remote preamp. Wish me luck!