Hi,
I have a pair of Guarneris and I love tubes and here is my experience in order of most favorable to least favorable. I own or have owned the following amps with the Guarneris.
Favorites
Berning + CAT Ultimate 2 (Berning needs the CAT - the CAT is by far the most synergistic preamp I've had in my system)
CJ16LS2 + Art Audio Quartets with KR845 tubes
Medium
Joule Stargates (lovely triode, but not enough power for the G's)
Conrad Johnson MV60 el34 tubes (with Winged C or Sovtek) - not nearly as much bottom as any of the above (or as a Tom Evans Linear A on loan), but lovely midrange.
Least
EAR534 (slow, muddy and poorly machined)
Conrad Johnson MV60 (ei tube or svetlana "s" tube)
The Guarneris are also very sensitive to the source. I'm currently using an Audio Note DAC and CDT2 and it sounds wonderful. I've had medium results with a Wadia 301 and poor results with an Arcam fmj23. For vinyl I've had good results with a KandK phono kit and poor results with an EAR834P.