Wow 604 man ,please give some details on that and what tuning the box fb and f3 is ,I remember the honorable altec man Greg Montfort or GM on the altec board had some big boxes and mass loaded designs,
The big boxes I remember were usually the urei copies with the 604 and a helper driver from jbl,gauss,ev,Martin,etc
The box dimensions are 33in X 53in and 21in deep. 33X53in is the Golden ratio which is has
been used in architecture and design for centuries.
The cabinets were built using Baltic birch plywood made in Russia , that comes in 5x5 foot sheets, 3/4in thick. Bracing is edge mounted Baltic Birch, front , sides and back. no problems
with flexing.
There is fiberglass padding on the top inside, back and one side.
The duct is made out of PVC pipe 6in diameter and as I recall, about 8in long . I have A PVC
coupling that I can put on the the end of the duct for a tune-able response.
An ideal tuning is the inverse of your room gain , which is probably +3 to +20 DB depending on your room size and leakage.
The center of the horn on the 604-8G is 45in above the floor which puts the 604 at ear height
when seated.
Subtracting the 604, braces & crossover , Vb is about 20.5 cubic inches which is close to the
Vas of the 604-8G.
The honorable GM drew the line at Vb = Vas , this could be a rule of thumb .