Is my ceiling too low?

Shouldn't be a problem, but an effective solution would be to use speakers that have limited vertical dispersion. Tyler Acoustics' new "Pro Dyamics" series comes to mind - very intelligent configuration in my opinion. If I was designing a speaker specifically for home theater, that's the configuration I'd use.

Duke

I'm having great success in a room with a 7'6" ceiling, but also much longer and wider (27'x18').
.

I had a room with low ceilings with reynaud offrandes, one of the best sounds I have experienced. As I remember the tweeter is below the driver on that model which may have something to do with the results.

Do you bump your head?

In all seriousness, I have a soffit for the central heating/cooling system ducts that is 5' wide and runs the width of my room directly over the sweet spot. It reduces the ceiling height to 7'4' in this area. This has never presented an acoustic problem in the 20'x 28' basement room.

I had a similar but worse problem! My suspended ceiling in the basemant was 6'-9". I took the suspended ceiling out, put acoustic panels between the ceiling joists, and covered the bottom of the joists with fabric. The fabric is a little loose, and has a small drape to it. It looks cool.

What this accomplished was to give me an effective ceiling hieght of almost eight feet, but still giving a finished look. The fabric is about 7' off the floor, but there is nothing preventing the sound from traveling up the full hieght of the under side of the first floor.

Another thing I did that really helped was to move the speakers into the corner so that the ceiling joists are running at a 45 degree angle to the speakers.

Once a basement listening room is finally set up they can be great, but it takes a while to get there...

Sure, the higher ceiling would be better but the Harbeth's are not very room sensitive and you will get some really great sound pairing them with the Naim stuff--provided it is the NAP200 or better. Enjoy!

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.

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!

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

Your lower ceiling is only a problem in that you're getting more reflection in time vs. direct sound from your speakers, all things equal. If you sit closer to your speakers, this isn't an issue so much. Also, more "controlled focussed speaker designs", dipoles, Dappolito's, etc, tend to cancel out off axis (vetical) frequencies some what, minimizing ceiling/floor reflections...this helps. Otherwise, you need ceiling treatments more likely than higher ceiling. Again, if you sit closer to your speakers this negates these issues quite a bit, and helps.
On the other hand, if your ceiling height ratio,vs one of your length modes is within 5% of each other (7.8 and 15.3 in your case), you'll also have doubled up bass modes in certain ranges, which will definitely compound problems acousticall as well!! Proper speaker/seating becomes even that much more critical, and even some of this new EQ stuff on the market, like Rives PARC, or more DSP bassed stuff that's now arriving. Good luck

You are right, I didn't notice the length/height is so close to 2! This will be a bigger problem... Anyone can tell which case is worse: length/heigth=2 or length/width=2? or the same?

I have a family room (LxWxH=19x13x7'9) in the basement can be used as a hifi room too(but will cause more money to build two doors to close this room. Is this room better than the other one(15'3x12x7'9)? Is it too big for my system? Thanks.