Bookshelf Speakers that have to be up against the wall

   You've had some pretty good suggestions here. Speaker placement is paramount to making speakers sound as good as they are capable. Sounds like you are restricted to placing the speakers so close to the wall. "Bookshelf" speakers that are rear ported usually have that feature to enhance bass. To calm that interaction with room boundaries (walls etc.) some have plugged the rear ports to minimize close wall reflections. Of course that minimizes the bass response the speaker was designed for. Some add a sub to compensate for the loss in bass.

   You didn't mention the particular speakers you have? Adding sound absorbing material to the back wall or plugging the rear ports of the speaker may or may not improve your sound. You probably want to explore all options before shopping for new speakers. If you must place the speakers so close to the rear wall, shop for ones that are not rear ported.

    millercarbon is completely correct! All of those things suggested should be investigated. BUT!!!!!....Are you completely locked in to the placement of your speakers? If so, your options are limited. Even if you replace your existing speakers, the new ones will also offer a challenge as to their placement.

   It all comes down to what you perceive as acceptable sound for you. Your sound "theater" should be what you accept as ideal with what you expect. I still haven't heard what speakers you are now using? Not that it matters so much but it gives a hint as to how improvements may be accomplished.