Sub measurements
Fist, I think the ceiling measurements are irrelevant since you wont have your ears in that location. My understanding is that room treatments (bookcase, rugs, carpet, etc.) will help deal with the higher frequency room issues including the echo you described, but bass is more difficult.
Huge, ugly bass traps or expensive electronic equipment would be required.
Your best bet is too experiment with sub locations and positions in the room to see if you can get a better response. It takes time, but the result will be a less boomy sub. Assuming your mains play low enough, set your crossover at 80 Hz or lower to eliminate the bump on the higher frequencies.