Hard to say with all the variables. Put on some thumpy bass filled music at a decent volume, and go crawl around in the corner where your TT will go, and find how far out your head stops throbbing.
Or use an SPL meter in stead of your head if you have one.
Different low frequencies will have different boundary reinforcement, so try it with different stuff (or a sweep test tone if you have something like that).
BB