Higher powered but efficient amp with the imaging of a Pass XA-25?

If you are looking for more power and the warmth and sweetness of Pass, I would suggest AVM.  They use a MOSFET output stage that is brilliant and offers the kind of textured sound you are looking for rather than the hyperdetailed cleanliness of a Class D.  

The AVM Ovation SA 6.3 will give you 225w of power and is a brilliant match for those Kefs at $10.5K.  

You can get a lot more power out of the Ovation 8.3 line and it has a tube input stage that uses ECC83s but it is much pricier with the 450w SA8.3 at $20K.

Another brilliant alternative is Bricasti but to get the power output you are looking for you would need to look at their monos which are $30K and $33K respectively. 

I am a dealer for both of these brands so I am undeniably biased but I do think they will work for you and resolve your problem.  

@verdantaudio Thank you for the suggestions. After I incorporate the RP-7 and explore its capabilities I will investigate the AVM line.

Maybe Luxman?  I’d be on the phone with Clayton Audio to see if they build a balanced version of their S100 amp.  Probably as good or better than Pass, and although it’s a Class A design there’s a high/low bias switch so you can leave the amp on without heating the house or running up your electric bill — kinda the best of both worlds. 

http://www.enjoythemusic.com/superioraudio/equipment/0314/clayton_audio_s100_power_amplifier.htm

The Plinius SA100 Mk3 operates similarly, and there’s a nice used one available at what I think is a bargain price if you’re open to used. 

https://www.usaudiomart.com/details/649797665-plinius-sa100-mkiii-class-a-ab-power-amplifier-updated-and-upgraded-recently/

Just a few other ideas to ponder FWIW. 

I agree with @ghdprentice . You are all over the map You have gone from 25wpc to 250wpc and from 97db floor standers to 85db bookshelf speakers. You are trying to fill a big room but that won't be done with bookshelf speakers. Changing amps isn't going to change that fact.

I too have a big room which is also my LR, & DR which is 15 x 32. I fill it with a 65wpc Don Sachs Kootenay and Silverline speakers with subs..Before that an ST-70 w/ 35wpc.  FWIW Don Sachs also has worked with Spatial audio and has their speakers

@russ69 Your suggestions?

Time to shop for new loudspeakers.