Is my ceiling too low?

If you have problems, it can be hard to sort them out all at once. Place your speakers for maximum bass smoothness, then sit in your listening seat. Have someone get a ladder and a small mirror and move the mirror across the ceiling until you can see the front of the speakers in the mirror. This is the first reflection point. Often a bit of treatment at this spot will break up the first reflection and that can get you a good ways to improving the issue. You may have to experiment with how much area needs to be covered as this is a function of the dispersion of the tweeter.The same technique can be used on hard-surface floors and rear and side walls.

Thank you so much for all the helps! Now I know there are still many things that I can do to get a great sound in this low-ceiling room. I think the first thing I would do is to ask my builder where the duct will be in this room and how wide and deep it is. Now I have another question, if you have a choice, where do you want to have the duct in a room like mine, close to your speakers or close to your chair? Thanks!

Add some diffusion on your ceiling. Auralex makes some excellent diffusors. I have a similar basement ceiling problem. As Viridian noted above find the first reflection points and use a combo of diffusion and absorption panels. This can really make a differnece and overcome some if not most low ceiling problems. Look at my system for some ideas if you want. In most cases you wouldn't have to go to the extreme coverage like I did, but I have a dedicated room, and I please myself, this arrangement sounded best.

7'9 is still pretty tall, actually insanely tall for a basement. I think you'll be fine.

you're fine....don't hit your head