Soundstage and the effect of the rear wall

First of all, are your speakers ported? If they are rear ported, you have problems. Many speaker companies have made speakers that are meant to be bookshelf or wall mounted (Soliloquy Sat-5).

You will get better soundstage by moving the speakers out into the room. Since that isn't possible, I would recommend several things: 1-make sure that the speakers are the furthest item sticking out from the bookshelf-you don't want stuff (including the shelves)to block the sound waves. Very full bookshelves will help also. This will eliminate a lot of reflections and help eliminate vibrations. Finally, if you want to go hard core, cover the rear wall with soundproofing material.

I agree with Elevick, but want to emphasize that getting great sound isn't always the most practical. Speaker placement is the most important issue relative to almost any tweak/isolation move you might make.
To know for sure, try borrowing some decent speaker stands 24-30in height, and listen for a weekend with speakers in optimum position. Check FAQ(room acoustics) or rives.com, for detailed advice, but I'd try 3-4 ft. away from the wall, as a guesstimate. If it doesn't work, fine..,no worries; if things sound great...then you've got some thinkin' to do.
Good luck, Spencer

A flat but precise soundstage can be effected with bookshelf (or other close-boundary support) mounting. However, soundstage depth can't really exist behind the speaker plane except via frequency-related nonlinearities (psychoacoustic tricks). Speaker designers have a major decision to make when designing for free-space or boundary support, irrespective of venting issues. A good example is how JMLabs and PSB both blew it by simply taking their normally stand-mounted satellites and reconfigured the cabinets for surround function, but seemingly didn't bother to revoice them for the consequent boundary support. The result are lots of congestion in the upper bass and lower mids. Further, Boston did a great job here with their VR-MX sats, as an example. I just saw newish versions from Spendor and Wharfedale, and trust that these surrounds are properly voiced too. As a gross generality, if a small speaker sounds too lean on a stand in free space then it was (hopefully) voiced for boundary support, and may sound much better on a (sturdy) bookshelf or on a stand pushed closer to the wall. OTOH if it sounds great in free space it risks sounding bloated, congested and too warm with boundary support. Thank you Roy Allsion!