This could be a list of the hardest to find preamps if you consider the USA market. FM Acoustics has one dealer in Colorado; Jadis is not distributed in the US as far as I know, although you might get it from Canada and the Viola has perhaps three dealers? The chances of hearing any together, particularly new or demo items is pretty remote. I would say that the Jadis looks like a sonic outlier relative to the other three. I've owned the Criterion and it is a big improvement in clarity over JRs previous preamps (I've owned Coherence II and Synergy). I would consider Viola if I could arrange a local demo and also would put Jadis on any short list of all tube systems to build.
As for FMA; I've heard the full FMA setup, met Manuel Huber (a very nice, knowledgeable and enthusiastic person) but it doesn't make my list of things I'd someday like to own (realistic or not).
As for FMA; I've heard the full FMA setup, met Manuel Huber (a very nice, knowledgeable and enthusiastic person) but it doesn't make my list of things I'd someday like to own (realistic or not).