Normally speakers are desingned with the entire stud cavity as the acoustic chamber, and yes, you will probably tighten up your bass with the addition of some fiberglass. But in all honesty, you will never acheive the performance you desire unless the in-wall/ceiling speaker includes it's own acoustically designed enclosure. A couple names come to mind here POSH, and Monitor Audio. I'm sure there are more upper mid end speaker manufacturers that take this into condiseration. Putting this in a retrofit application will be a challenge for most...

Hope this helps.