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