Best Tube Amp For Sonus Faber? Or Pass XA-100.5? $6-7,500 Budget, Purchasing Used.

  I don't know the rest of your system but synergy is very important. Also keep in mind that the preamp has a bigger impact on the sound then the amp.

@lostbears , keep also in mind that it's not important the level or class of preamp. more importance plays input/output characteristics of amp/preamp in terms of sensitivity and impedance.

Personally, I haven't heard any Sonus Faber speakers sounding better with tube amp vs. solid state. Mentioned ARC reference mono amps probably would do the best job. VTL or Manley labs also make a line of tube powerhouses, but thinking of heat they generate and upcomming July already making me sweat or setting my AC on maximum during playback...
The combo that impressed me with SF Olympia is Classe CP-880/CAM350 monos. I don't think you can beat that with tubes if same funds spent.

Similar situation with Avalon Ascent MK2 speakers that have somewhat similar characteristics and class. It was sounding clean and precise with BAT VK150 monos, but Pass XA250 simply added BODY to the sound and even perhaps spirit which tube amp couldn't. 

So if there's a choice bettween 

ARC Ref 150
VTL MB250
Pass XA250 

I'd pick pass without doubt 

Spearit Audio has a new CJ 125 SA for $4999. Makes a nice match w/the Sonus Speaker.

Would not go with the VAC Phi 300.1.   Definitely go with the updated VAC Signature 200 IQ with continuous auto bias.  100+ watts and has all the bass and tightness of solid state and the midrange is fantastic.    Retails for $14k but there is a like new one listed on US Audiomart.  With the auto bias of the 200 IQ, you can plug in any KT88, KT120 or KT150 and change out the 6sn7 driver tubes if you want to tailor the sound.  It is so user friendly and the stock tubes are wonderful though.  Have owned both the Sonus Faber Olympica III and the Elipsa SE.  The VAC controls the bass of the olympica which can often sound a little bloated if not having a high quality amp.   If I were to go tube amp for SF, would go with the VAC 200 IQ.    Plus Kevin Hayes of VAC offers the best service around.  You are kind of limited with other tube amps (ARC, CJ, Zesto, etc) as the VAC is really user friendly.  

Here is the link to the VAC 200 IQ:  http://www.usaudiomart.com/details/649357991-vac-signature-200-with-vac-iq-system-power-amplifier/

Here is a forum from a dealer on the amp as well:  

http://www.audioshark.org/vac-85/vac-signature-iq200-9738.html

Keep us posted!

I would go with the Pass, as the  Olympica III in the power region is around 2ohms at 100hz, a tube amp would be a gamble.
https://ibb.co/hoYNyF

Cheers George