The Diamond series is identical except the woofer size. This improves extension on the lower end as you go up the scale. If you are looking for a purely 2 channel music setup without a sub, I'd go as far as the 9.2 to get a good range overall.
For 9.0, I think nearfield listening is gonna be completely devoid of bass, positioned close to the walls, there might be ok bass for music, but certainly nothing you can feel as such.
9.1 is gonna be somewhere in the middle. I havent heard it myself but I would expect it to have a bit more bass compared to the 9.0
It comes down to musical tastes and what you expect. I personally like my bass, but don't like boom, and don't listen to anything bass heavy like Hip hop, trance, dance, techno etc etc. Mostly Rock, metal and some jazz classical thrown in. If your tastes are similar, the 9.1 or even the 9.0 might be ok.
I have TEAC speakers with 4 inch drivers that I am very happy with for music without a subwoofer simply because of its strengths in all other areas and the reasonable bass that I can hear but not feel as such. They are again not great for nearfield listening, but positioned correctly, you can get decent results.
