box building is driving me crazy

Yes, if you take a length off one side and add it to another side the net volume stays the same.

The gross volume with the dimensions you give is 2.06 cuft (assuming 3/4" MDF construction.)  So you should be very close to 1.5 net after mounting your sub and subtracting port and bracing volume.

No, actually it does not stay the same. It will only stay the same if all sides are equal. Here is what I mean based on your measurements.

23.25widthx13.5heightx14.5depth    

I am going to base this box on the premise of 3/4" mdf used for all surfaces, 2 surfaces = 1.5" of unusable airspace, this must be subtracted from the measurements.

So internal airspace is 21.75" x 12" x 13" = 3393"^3 or 1.96ft^3. This is your boxes internal gross airspace using your measurements.

If you take the 2" off of the box the way you are suggesting, here are the measurements based on this new equation.19.75" x 12" x 15" =3555"^3 or 2.06ft^3. So a little simple math shows that 2" here or there can make a difference when it comes to box design. Dyohn must have been asleep in math class today or maybe my math is wrong (I'm sore today....road hockey champs.....yah baby yah..who wants a face full of bark mulch....)

Yike, I flunked today's math test. Thanks for the catch Rob.   I will beat myself with a large Canadian bacon. 

No offense intended, but by the sounds of it, you are attempting to build your own box for two JL 10W7's.... the box is more important than the sub itself. If you were to build a box and I were to build a box, there is one thing that I know is for certain between the two. Mine will come out the winner hands down for build quality, spl if is a spl system or sq if it is a sq system. Don't take this as a knock on your capabilities, but for the single most important part of a sub system using such a nice high end driver, have the box made by a professional please.

I will do that at the next Sharks game.  IF there ever IS a next Sharks game.