Looking for recommendations on Tube AMPS to drive Monitor Audio Platinum Speakers


I have Monitor Audio Platinum 100 ii Bookshelf Speakers. I am Interested in anyone who has these speakers or heard them and can offer insight into which amplifiers may have driven them well. My preference is for Tubes and I have an Ultravalve (ST-70) that does not always seem to be up to the job. My prior Monitors were Sonus Faber and were easily driven by the AMP but these only some of the time not always. I will listen at higher volumes ¼ of the time.  Enjoying Rock (Classic), Jazz and Jazz fusion.  My Preamp is an Atma-Sphere UV-1.  I use Morrow Audio Cabling PH-6. Vinyl listening is preferred. Acoustic Signature/Ortofon-Quintet Black/OrtofonVertoSUP. For Digital I listen thru PSaudio Perfectwave Dac.


Mark

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Your speakers measure(from SoundStage)86.5db.efficient,nominal impedance is 8 ohms,they never drop below 4 ohms BUT they have several MAJOR phase swings(thats why your ST70 doesn't cut it)..ANY good KT88 driven,Class A Push Pull from Cayin,Rogue Audio,Icon Audio & about a half dozen other makers should make sweet music.I suggest something beefy like the VTL "VT85" or similar.
I’m curious WHY would you go from Sonus Faber,who I consider one of the top half dozen speaker makers on the planet,to Monitor Audio who are NOT?

thank you all so far

@glow_worm thinking the AMP has 36 watts of tube amp power  it met the low end if doubled?

@jjss49  yes AR is in the house ( on paper only so far)  so to speak for reviewing

@freediver  excellent question . also the impedance on the MA site has 6.5 OHM - so I found that odd.  My Sonus Faber were 20 YO Concertino . AS sweet a sound as they were - they were small and not always filling my 12 x 15 room. I have a B&W sub which helped with the bass. Having had them for 12 years I was ready to try something new. I looked at both new and used in the $2-3K range . was ready to buy the DYNaudio 40's. but the MA were available local - used 2 years old for $2K and said to myself why not. I could always sell them if I do not like them . The AMT tweeter and the woofer sound excellent . So I am keeping them for now .

I would agree with the above that a 6550/KTXX tube would do well with your speakers.  I had a pair of VTL MB-185s that I think would do well with those speakers.  You should also look at Rogue as they make fine amps.  I personally own a Hydra which is a hybrid tube/Class D. 

Otherwise, if you have an Atmasphere pre, seems obvious to look at an Atmasphere amp.  
What sound/performance characteristics are most important to you and what do you like?