I agree with what has been said so far. Jeff is right. Start with the suggestions made in the manual and work from there. If you don't use a room eq. device, I heartily recommend you purchase a radio shack sound pressure level meter and a good test cd. This really helps in adjusting settings to adjust for those peaks and nulls.
I have my Descent set up about four feet from the front wall and one foot out from the side wall, in a twenty by twenty-two foot room. I have my phase set at 90, my level at 5, my low pass filter at 70 Hz. (which matches well with my Ascents) and my 25 hz. filter at -12 db to make up for a large peak I had at that level. I arrived at all of this by starting with suggestions in the manual, and then just playing with various settings and locations and seeing how each affected s.p.l.s with the test tones. I now have pretty flat response all the way down. With the Ascents bi-amped, I get very pleasing bass response for music and movies.
Take your time to tweak it and enjoy the process. That is part of the fun of hi fi -- playing with the gear and getting it just right for YOU.