I have owned the A21+ and liked it. It was a bit too warm for me but I thought the Benchmark LA4 was really good with it. I tend to love the LA4 (or HPA4) with all the amps I have paired it with. I use the HPA4 with the CODA #16 amp (not selling either one).
A few weeks ago, I sold the Holo Audio Serene preamp which is a bit like the LA4 but warmer. Today, I got delivery of the new Schitt Kara F preamp. It cost $799 and going into my office system where the HPA4 used to reside when I had the Serene preamp.
I cannot tell on the first day with the Kara if this is a killer preamp. So far, the Holo Serene and the HPA4 are better. I have a feeling the Kara will end up sounding like the Serene. All 3 measure about the same.
I would have bought a LA4, but I did not want to spend any extra money and the funds for the Kara are coming from a future sale of my Schitt Mjolnir v3 headphone amp + preamp. The MJ3 is very warm and rather good and in demand.
My new Kara F is going into the new Schitt Wotan amp driving Magnepan Mini speakers. The Wotan is really good. After a few months now with $2k Wotan I think it is good as the CODA #8 that I owned at one time.