It's sealed, so the audible variations are sublte as you vary stuffing density. If you put 4 lb. in 4 cu ft, you have a 1 lb.cu ft density - speaker level of fill. Subs are frequently stuffed much denser; Dickason's LDC shows data for a variety of stuffing combinations and density levels, up to 4 lb.cu ft, with benefical impact on LF response. Conversely, Bill's aiming you at a stuffing density that gives the most linear response, 0.7 Qtc, and my simulation says that requires "heavy fill" so I doubt you'll add too much!
When starting with an undersize box (empty Qtc >>0.7), too little results in boomy bass, too much... hard to have too much as response just get smoother below 0.7...
Since I have a Umax 15 simulation, I've attached Unibox results for "light fill" (which is less than "walls covered?") with Qtc = 0.84.
and "heavy fill" with Qtc = 0.71
Fb is lower with heavy fill, but F3 higher... light fill has a 0.5dB peak at ~60Hz that's potentiall the most audible aspect... higher Qtc level become quite peaked and audible. Like I said, subtle...