Yeah, I think I have commented on this here before, but I don't remember where. So I'll keep it brief:
I found the AD30VT's stock speaker to be kind of harsh at the top end, and the bass tended to be quite loose/raggedy when pushed. This was most noticeable on the higher distortion amp models. The Celestion G10 improved both; the highs are definitely smoother and the bottom end stays much tighter and better defined. I haven't noticed the boominess with the back enclosed, but then again I play at very modest volume levels.