I've always been impressed by how good Bill Clinton spoke when he was in a tight spot.
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Most speakers do not like to be placed up against a rear wall. Bass radiates in all directions (front port, rear port or no port still radiate in all directions). These setups give quarter wavelength cancellations (comb filtering) from the rear wall reflections in bass frequencies.
1) Nearfield arrangement: place your speakers well out into the room (about 5' out from the 8' rear wall and about 4 feet apart (2' from side walls) and sit 6 to 8 feet back from the speakers to listen) Toe in as necessary to reduce side wall reflections. A three way with decent bass would help in this configuration....maybe a tower such as a Veritas 2.4i.
2) Farfield: Use toed in satellite monitor speakers in the corners of the 8' wall with a sub for the LF placed one third of the way into the room. The sub should help even out the bass response (quarter wave cancellations from main speaker rear wall reflections) provided you don't filter them more than 12 db/octave above 90 Hz and you are able to find a good position that is not too modal. (i.e. at 180 Hz you still have some decent energy from the sub)
3) Soffit mount. Since 20' gives you plenty to work with you might consider placing the speakers inside the 8' wall or bringing the 8' wall into the room two feet to create a cavity to house soffit mounted speakers...Paradigm makes some specific gear for in wall installations - so does B&W. (Note that soffit mount eliminates comb filtering of the bass by removing the rearward energy - it also boosts bass response by about 6 db which may limit your speaker choices to monitor type speakers)