Bryston Amp Experience Needed

I’m not sure what the sensitivity (efficiency rating) is for your KEF speakers, but my guess is that the Bryston 3B3 will perform fine and sound great in your system. 

I have a 4B2 which is a wonderful amp but I had an in-home demo of a 2.5B3 with my difficult to drive Thiel CS 3.6 loudspeakers and OMG the power was fine with my stubborn speakers. The sound quality compared favorably with my 4B2. A slight lack of detail and me having to strain to integrate harmonies from musicians interacting with each other and background vocalists were better integrated into the overall sound for the 2.5B3. 
For low to medium volumes the 3B3 would be great. Now saying all I have the 4B3 would be greater-er! To be honest, I have not yet auditioned the 4B3, but I have such trust in Bryston, that when I get ready I plan to order one sans audition.

I would lose a few pounds and go with the 4B3 for the reasons that others have already mentioned. 

I am a big fan of the 4B3. It truly is a great amp. But what is the sensitivity of your KEF speakers?

I moved into a new house which features one large multi use space. Has tall wooden sloped ceilings. 
I brought my loved Rega Elex-R and Rega Rs7 floor standing speakers. My source was a SONOS Connect but I upgraded that to the new SONOS Port. The new living space sounded not so great. I swapped the Rega kit for new Spendor D9 speakers, a Pass Labs XP-20 preamp and a Bryston 250 watt 4BSST-2 power amp. I dumped the SONOS Port and added a Linn Majik DSM streamer as my source. I also have a great Rega turntable station.

Playing favorite records at thumping volumes sounds great. Some digital still sounds too cold however. I talked with a bunch of people and the advice I got generally is that the Bryston is not the best match here at the 40-70 dbls I often listen to? If I crank the volume the Bryston totally delivers, big time! On some recordings at lower volumes I am not as happy. 
My plan is buy a Pass Labs amp and see how it performs at different volumes with different sources. I am told this will be a better match for me than the Bryston? 
Wish I had already done this A/B test and had real findings to report for you. I hope to get this done in the next couple of weeks... 

I use a 3B² with Focal 948's. Wonderful amp. One of the best companies in the market place. you will get the balance of the warranty as well. Buy the 300W if you can.