I ran a C370 with Diamond 8.3s a few years back in a budget system (with a NAD CD player originally, then a Rotel RCD-1070). this would be a similar sound to what you'll get from the C370 and 9.1s, though the 9.1s are likely to be more detailed yet have a smoother top end than the 8.3s but have less punch (floorstanders versus standmounts).
the C370/Diamond combination was an extremely good budget set-up. the C370 is big, powerful and smooth sounding for a budget amp - which is a good compliment to most modern speakers which (to my ears) are voiced with too much top-end emphasis.
I my opinion the C370 is very, very good. one afternoon, I spent some time with another stereo hobbyist switching the C370 in and out of system running Totem Hawks, and Audible Illusions tube preamp, and a good CD front-end. the comparison amps were a Bryston 3B-ST and a hybrid Counterpoint NP100 (a superb amp). Honestly, the NAD wasn't embarassed. It was obviously not as good, but it by no means sounded lousy by comparison or made either of us say "yeah, that's no good" Instead we were saying "wow, the NAD sounds good"
So for upgradeability reasons I'd go with the C370 instead of the 320 (more power and better ability to drive difficult loads gives you more flexibility with future speaker purchases - and the voicing of the C370 (big and smooth) probably makes for easier compatibility with more speakers too.
I'd suggest maybe stretching the budget a little bit, because the C370 can be paired with very good speakers without embarassment. Maybe something like Monitor Audio silver 8i's on the second hand market, or their Silver S6's. I've heard that match-up (NAD C370/monitor Audio) and it's spectacular budget sound
