You're right. Putting a cabinet inside a cabinet. Let's see, we build a cabinet in a way to reduce resonance and then put it inside another resonanat cavity? Not to mention the low frequency gain you get because the speaker is not out in the open. The result will be a very congested sound. I've always wondered--why do they call them bookshelf speakers when the worst position would be in a bookshelf?
If you want that "flush" in the cabinet look, then buy a speaker that was designed to do that from the ground up and build the cabinet to house it appropriately. Triad is one brand that comes to mind that has in wall speakers that could be used in this way. There are many others that I'm sure are quite good as well.
If one does put a "bookshelf" in the bookshelf, then line the sides with fiberglass--tightly at that, and you will need to eq the bass because it will have an unnatural gain.
If you want that "flush" in the cabinet look, then buy a speaker that was designed to do that from the ground up and build the cabinet to house it appropriately. Triad is one brand that comes to mind that has in wall speakers that could be used in this way. There are many others that I'm sure are quite good as well.
If one does put a "bookshelf" in the bookshelf, then line the sides with fiberglass--tightly at that, and you will need to eq the bass because it will have an unnatural gain.