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 .
