The basic issue is that most speakers are not designed to load into an acoustic half-space and that creates a rise in low and mid-bass most will perceive as thickness or muddiness. Proximity to the rear wall also reduces reflection time from the back wall which tends to reduce the sense of spaciousness and depth of the stereo image, AKA soundstage. The original Boston Acoustics A-100, a notably flat and wide speaker, was designed to be placed against the back wall, and used that to good effect. Predictably, they sounded thin - lacking in bass - when pulled away from the wall. Many speakers use rear ports to augment low bass, leveraging the proximity. Similarly, their rear-ported bookshelf speakers only need an inch or so of clearance function as designed.
The other issue is room size. Most bookshelf speakers will have a peak output of around 106-108dB @ 1w/1m. That sounds like a lot until the inverse square law kicks in and output drops 6dB every time the distance doubles. So, at 4M (13 ft) their max loudness is 96 dB, and max average level (general rule of thumb) is 12dB below that or 84dB. Not very loud. Somehow you need to find at least 6dB of peak output or 114dB to really work in your room. All that points to JBL and the 4329P powered 8". It meets the size, placement, and out requirements for your room better than anything else available and if you haven't heard JBLs in awhile, these are not the boom/shriek JBLs of old. Checkout the reviews.