I owned the A23 for a long time (used with KEF LS50’s) and it did the job, but I was not crazy about the sound. I recently bought a mint used A21+ and was happy with the results.
I preferred the A21+ over the A23 by a huge amount. I used the A21+ with a hard to drive Thiel CS3.7 and also my RAAL SR1a headphones. I got the A21+ mostly for the headphones, since I had a CODA #8 sounding great on my Thiels.
Since the RAAL SR1a are so resolving I noticed that the amp was very powerful and took a bit of the bright bite off the SR1a, however, there was a bit of clarity that it lacked. The A21+ sounded similar to my old CODA CSiB integrated amp.
I decided to spend too much money and replaced the A21+ on my headphones with the KRELL Duo 175XD. Now this was a homerun with my headphones. About a month ago I switched the KRELLL to my Thiel and the CODA #8 to my headphones. The KRELL has now become my favorite amp on the Thiel’s. It is the 175 watts of Class A on the KRELL that is blowing my mind. Just an incredible amp.
Take a look at this amp. It is more powerful than the Duo 175XD I purchased.
Krell Duo 300 XD Stereo Amplifier Dealer Ad - US Audio Mart
I listen for a min of 8 hours (M-F) on the KRELL and CODA amps and while both are excellent. The KRELL is now my favorite in the office system.