I found the A21+ a bit warmer than my normal preference, but when I paired it with a super neutral preamp like the Benchmark LA4 I liked the A21+ a lot. I currently use the LA4 with another slightly warm amp (less so than the A21+), the CODA #16. A great pairing.
You can do a 30-day home trial of the LA4. Don’t let the price fool you. You will not find a better volume control.
Benchmark LA4 Line Amplifier - Benchmark Media Systems
If you want a bit more warmth with similar sonic attributes of the LA4. though with less preamp features, and maybe a lesser volume control, check out the Holo Serene for the same price.