[mehulved]

6230i - I wouldn't want to touch this one.
6630 - Forget this with the budget you have
6131, 6233 - Haven't seen these phones so I won't comment, but do they have stereo speakers?
6680, 6270 - IMO, you'd do better with an SE, any of these would be your choice if you are looking for a good selling price which SE phones won't give.
W300i - forget this one. mono speakers, screen quality is too bad.
K750i - Not worth if you have budget for W810i
W810i - will be a good choice and you like it too
D820 - No comments here

I have heard sound from Nokia 6270, it's amazing too, as are it's headphones. Comparable enough to SE phones IMO.
Do remember to shortlist 2-3 phones and try them out before purchasing one.