stuffing your sealed boxes

If you fill the box with the correct amount of polyfill after you make a box to the manufacturers specs or whatever you came up with, wouldn't that kind of negate the point of having a "right" number of cubic ft.?  I don't know jack about this other than a rule of thumb is 1lb for each cu. ft. of space.  It just seems like you would ruin all your hard earned calculations if you "added" space to the box.

The fill is included in your calculations.  When you use a speaker box design program, you tell it how you'll be filling the box and the program takes that into account.  See Unibox, WinISD Pro...  Also, most good speaker manufacturers will give a recommendation of box volume including a fill amount, such as fully filled, loosely filled, walls lined, etc.  If you're not using a design program, follow the manufacturer's advice for volume and fill amount.

This is a snapshot of a part of the Unibox program.  The damping factor shown here can be scrolled from no fill, minimal fill, walls covered, to heavy fill.  Each choice changes the absorption characteristic of the box with a given value for Qa.  The program accounts for this characteristic when plotting the response.