Go with the A21, if possible, just for the SQ purpose. I had demoed both and preferred the A21.
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Always go for more power. This can eliminate the need to upgrade and losing money when selling gear. FWIW there is a Proceed HPA-2 amp currently for sale(no affiliation with seller). IMO the Proceed is a better sounding amp then the A21/23. Might even be better then the JC-1 mono block(have yet to hear them). The Proceed HPA-2 is one best values on the used market. The only caveat is the age of the Proceed amps(10+ years old) where service "could" be needed.
You are better with the A21 for better and more effortless sound quality. It is a good match with the Kef R series speakers . I am driving the R900s with the A21 and getting very good results. Make sure you are using a high quality preamp. Also the speakers because of the transparency and resolution capabilities react to cable changes and are easily identifiable. Another amp that worked well for me with the 900s was Classe SA150 .