80dB at about 50Hz would be fine. I admit with single tones it gets real loud and the place shakes quite a bit. With that sub you were producing 98db at 18hz.....that's ground shaking.
You'd find with REW that the frequency sweep used for measuring is quite short and painless and a lot quicker than the single tones. You should use it.
The BFD will easily flatten that response, which is already pretty good.
brucek