Well, the buzz is very plausibly the lower action and looser strings separately or in combination. Changes in volume and tone are probably the same thing;- lower tension, different strings. If it was the bridge being undercut, you could just raise it via the adjusters and see if it goes away. The new bridge is probably fine.
Perhaps try substituting the old string for the buzzing one, just as a trouble shooting measure. If it helps, look into getting a higher tension string that matches the other new ones, or perhaps a higher tension set all across. Results are not guaranteed of course, but it might restore the lost volume, maintain the lower string height and lose the buzzzz. Hopefully without getting into the complication of dressing the board.
If there is a high spot as could well be the case, you will be able to detect it by pushing the string to the board in two places perhaps 8 to 10 inches apart. Check out how much space there is between the string and board as you press it down. If it is dressed right you should be able to do this all up and down the board without the string touching the board between the two points you press it down. It will be a very small space, but it should be there.
Watch the bridge alignment when you change strings and wack it back into position if it leans toward the FB.