I’ve owned both in the past but not used them together. Both are great sounding pieces, particularly the A21, and not at all analytical or harsh. BUT if you can score a used Rogue Athena or Hera (if a great deal) preamp it will sound much better yet than any 99, even super-magnum’d. Then you can also get off the trail-of-tears that is finding good sounding NOS 6sn7 tubes that are also quiet enough and don’t cost a fortune. And I wouldn’t bother with entry-level Rogue preamps like Metis or RP-1.
The issue with some Rogue preamps will be noise due to their high gain (the 99 has 23dB which is a LOT), but fortunately that A21 has adjustable gain controls to perfectly counteract this - so it should be a good match.
The only Benchmark gear I've heard is the old DAC-1, way back in 2000s when it was being hyped as possibly the best DAC ever. I thought it sounded dry and analytical, somehow boring and grating at the same time. The A21 with a good tube pre will definitely NOT be like that at all.
