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

a dream test would be to compare "live" following preamps:

-Jadis JP80 MC MKIII

-Jeff Rowland's Criterion

-Viola's Spirito or Cadenza

-F.M. Acoustics 255 or 266

I had only the oportunity to compare Jadis JP80 MC with Audio Research Ref 3 ... the last one had no chance!

I have heard the Criterion (beautifull) and Viola's Cadenza (incredible precision) but on different days in diferent systems ... I have never heard F.M.

Have you had the possibility to compare these models ?
which other one would you add to the list ?