One of the best high end sounds I ever heard were big MBL 101D speakers situate in a room that was hilariously small, truly "closet-like" and I think smaller than the OPs. But they were essentially nearfield, and the room well treated.
I've heard other speakers in very small rooms work well. It depends on the speaker (for instance, something like Quad ESLs won't have as much interaction to the side walls), how you can position the speakers, what type of room treatment you can apply if necessary.
My first thought for the OP's situation was similar to what other's have mentioned. Closer to near-field listening, and choosing an appropriate speaker for that. But he mentioned the speaker has to be placed near the back wall, so, really that's the type of criteria that makes answering the question a lot tougher.
I've heard other speakers in very small rooms work well. It depends on the speaker (for instance, something like Quad ESLs won't have as much interaction to the side walls), how you can position the speakers, what type of room treatment you can apply if necessary.
My first thought for the OP's situation was similar to what other's have mentioned. Closer to near-field listening, and choosing an appropriate speaker for that. But he mentioned the speaker has to be placed near the back wall, so, really that's the type of criteria that makes answering the question a lot tougher.