Best preamp: Viola, Jadis, F.M. or Jeff Rowland ?

Roysen, my belief is that there is no best just different and what one prefers in their specific set-up.

Among the few very best preamps I suggest you might add the new Viva Linea from Viva Audio and Lyra Connoisseur.
For me these 2 with Jadis and Kondo Japan are the top5 but different flavors.

The more neutral and more compatible of them all are the Viva and Lyra.

I have owned the Joule Electra LA 300ME for the past year+ and my search for a great preamp is over. How does it compare to the above mentioned preamps, I can't tell you. I have compared the LA 300ME to some other high priced pres and for my ear the JE would be hard to beat IMHO.

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).