I spent a couple hours yesterday, building backboxes, from above.
For installing backboxes from below, find and mark the joists (electric studfinder). Then decide which stud bays you want to use, that best line up with the downlights/can lights (the hardest part, IMO). Then, cut big holes in the drywall ceiling. Install 2 x 10 pieces (or other wood to match the height of the joists), perpendicular to the joists. I used angled wood screws to attach them to the existing joists. I caulked my joints, generic caulk from Home Depot. I just built the boxes square, 14 x 14 x 10".
Let the drywall guy repair the holes. Then, using your studfinder, find the edges of the backboxes. Install the speakers, or have the kitchen installer do it, if trust his judgement.
Electrician is coming later this week to move some of my can lights. I'll do the final speaker cutouts after he moves the lights. My new speakers are larger than the old 6" ones, so I have some play. Drywaller is coming after I decide on the final speaker positions.
You could do some research on in-ceiling subwoofers. I haven't used any for distributed audio, but I've read it's certainly the best way to get good bass for whole-house audio.
When searching for trimless speakers, also use the search terms 'narrow bezel' and 'disappearing'.
Many speaker manufacturers use it as a feature of their higher priced models, but even Monoprice has trimless grills now.
Edit - I don't mean to imply that you don't know how to build backboxes. I just thought this might be a good place to post this info, for posterity.
If I had to build backboxes again, for these speakers, they would have been rectangular, and larger. They are currently about 1.1 cubic feet.