Ceiling acoustics

Can anyone tell me how uneven ceilings affect acoustics? My (recently-expanded) listening room is in the basement, and the ceiling is crossed by two beams and ductwork. The room is about 13' X 27'. The ceiling is about 8' at the tallest and 7' at the lowest (i.e., at the location of the beams and ductwork). The first beam (about one foot wide) is at about the 12' mark (crossing the longest dimension), ductwork is at about 15' mark (and it's about 3 feet wide) and the final beam is at the 20'-21' mark. So, the ceiling goes up and down. How will this affect the sonics and what can I do to remedy any problems? (We move in on 3 Feb, so I can't listen and see what it sounds like.) Thanking you in advance for your responses!
Actually, irregular ceiling heights help to keep standing waves from forming. The beams won't have much effect (unless you hit your head on them) but I'd recommend you center the speakers under the 3 foot wide duct facing the longer side. You also should deaden the sheet metal duct using that rubberized material along with more stiffening to keep it from sounding like you have a kettle drum in the room!
Thanks, Nsgarch. Perhaps I missunderstood your suggestion, but I can't aim the speakers at the long wall, because I have to sit at least 10' from the speakers for all the drivers to integrate (first order crossovers) and the room is only 12' wide. The beams/ductwork are all behind drywall, so I hope that deadens the "kettle drum" effect somewhat. :-) I certainly hadn't thought of that ...
Fairley, what I meant is that (from your description anyway) the duct crosses the room's 12 foot dimension leaving 15 feet of room on one side and about 9 on the other + 3 ft W duct = 27 ft.

I was suggesting you place the speakers under the duct facing the 15 foot deep part of the room, that is if I understood your description of the room correctly.

What kind of speakers will you be using?
I've got Thiel CS2.2s now. A friend has a pair of CS3.6s that I will borrow, listen to and then buy if I like them.

I think I misled you with my description, or rather, my description of my room wasn't very good. The distance from the wall behind the speakers to the first beam is about 13'. Then, a gap of a foot or two, and then the duct begins, so it's at about the 15' or 17' mark. If I had the speakers that far out into the room, I'd have concerns about bass response. And I can't flip the speakers around to the opposite end of the room, for several reasons that I won't go into here.

Thanks again for your response. I guess I'll have a lot of trial and error listening for a few months. At least it will be fun ...