Cheap, small bookshelf design...

Flush mounting makes a big difference, I would definately do it. Unless of course the measurements you used to model your crossover were made with the drivers sufrace mounted, in which case I would stick with the original XO.

An easy way to test the flush mounting would be to get some felt or cardboard of the same thickness as your tweeter flange, and make cutouts in it in the shape of the drivers, then put this on your baffle so the tweeter is flush with the felt. You could also do this with something hard and/or permanent like masonite, but it would be harder to get the cutout just the right size, the felt/cardboard would be a good quick and dirty test IMO.
Nm2285 said:
As can be seen in the last post, I'm wondering if flush mounting the speakers could help get rid of some of the tweeter sibilance that I've encountered.

Seems like you're looking at the wrong place to start.
1,9Khz crossover point is too low for that tweeter, you need to cross higher.
The sibilance you hear is distortion, that tweeter can't handle such low frequencies.