First off, i claim no expertese on the subject. I'll just tell you what i've done/seen that worked. what seemed like the easiest fix was carpet on the walls. it was done in patterns that made it actually interesting to look at as well as functional. Curtains work, but are expensive and involve the fire dept's thumbs-up. I have acustic ceiling tiles on sections of my walls that seem to work. obviously, the less "flat" (thus unreflective) you make a surface the better and the less parallel to any other surface is good.
it took me awhile to figure out the place handles sound differently when the seats are filled than when they are not, so i started adjusting sound when the place had peopel in it -- world of difference. unless you have a "no food" policy, carpeting anything other than the aisles strikes me as just a bad idea. i think that's why the slope of the floor contributes substantially to reducing problems of sound reflection from the floor.
there is a theater in Paris where they did the walls in cinder blocks, with the holes pointing into the auditorium -- the blocks stacked on their sides, not tops and bottoms. every second hole had an insert that made it a different depth than its neighbor. looked cool and worked great.
i am obviously no expert on this, but there might be an idea or two here you can run with. good luck. my acustic probelms are an ongoing battle.
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