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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@soix  so after communicating with Don he is advising me against his AMP as he is not convinced 65 TUBE  WPC will be enough to effectively drive them based on the speaker specs from the MFG . 

It may be time to look for different speakers :( 
Steely Dan is my fave group of all time.  Given what you’ve stated, if it were me I’d order a Don Sachs KT88 amp ASAP.  I think it will bowl you over, and he offers a trial period in the event it doesn’t work for you — but I’ve no doubt it will.  Only downside is there’s probably a delay before he can build you one so you’d have to wait a bit.  Some things worth waiting for IMHO.  Go for it!

thanks all so far

@verdantaudio I have been checking out the VTL offerings I will be checking in with Ralph but is seems some of the OTL's are not  so good with the major phase swings and if I do go with Atma-Sphere my fear would be I will need closer to the top of his range which requires a bit more $$$ and more tube maintenance

@fstein having a HQ Dynaco clone now so am looking to switch it up a bit . I do have some Klipsch Heresy's as well and will keep the "ST-70" for them.

yes @ebm VAC is on the list too.... along with Quicksilver.

@soix  that is the big question and one I find most difficult to answer.

"What sound/performance characteristics are most important to you and what do you like"

  Every change I have made the last 10 years has opened the door to Sonic improvement .  I hear more , the distinction of the instruments and the sound is often better - or I realize my vinyl is bad.. I have also found with the improvements other refinements are needed to compliment the recent change. Everything I have done so far made a difference ( not always better )  which has lead to another important - change/upgrade.  Cables, Cords, Electricity all included. Recently I swapped a phono-pre Black Cube 2 for the Verto SUP . the previously owned (Used) -Costs were roughly the same and I found the sound better - smoother - warmer - realistic to my ears.  I have also heard at times some albums that once sounded OK - not sound so good anymore  .  Now I find I am replacing some of my vinyl.  To try and answer you , I prefer warmth with a good soundstage . The tubes have shown me the  warmth and the slightly larger more dynamic woofers with the  AMT tweeter opened the Soundtage for me along with adding bottom end  . I do not care for brightness as I find it to fatiguing after a few songs . I have some Celestion A3's in my home theater . They can sound awful with certain tracks or movies  and I use a Pioneer Elite system - all ~ 20 years old. My Kipsch speakers  when driven with the Rogue Audio Sphinx v2,  which I recently sold was too bright for my ears. I suppose complete 100% accuracy is not that important to me . But the instruments must be discernable . I need to hear the acoustic guitar when played and the subtle movements of the hands moving up or down the neck. I want to feel the urge to get up and dance when I put on some Reggae. A Strat by Clapton needs to sound like it should with great tone and a Tele played by Roy Buchanon etc...And when I listen to Ben Webster the sax needs to be in the room . Steely Dan needs to sound exceptional . I hope that helps . I realize I may be looking for the Swiss Army knife of Tube Amps at a reasonable costs . But a neutral , clean , warm  quality sound is what I am looking for . An Amp that won't cost an arm and a leg to re-tube and ideally I can do some minor tube rolling for fun. If it needs work it should have some simplicity .

What sound/performance characteristics are most important to you and what do you like?
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.  

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 .

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?
80-100 wpc audio research tube amp pick your model depending on your budget
My experience with MA is  the minimum power requirements listed in their specs are quite accurate. The min on this speaker is 60w, so I would definitely use this # as a guide.