A couple of years ago I built (and subsequently did everything I could think of to) a pair of 825 cabinets.

Realize that this cabinet pre-dated T/S, was essentially never revised with respect to sonic performance from the time of its introduction and was primarily intended to wring the most sound (not necessarily the most accurate bass reproduction) from low powered tube amps. It has very little bass ouput below 60 Hz in its stock form (using single frequency SPL meter measuremets).

Many experiments, but without the aid of RTA which I didn't have at the time,indicated that a port height of approximately 3" was optimal with the 515-G. You might do some searching on the Altec board in this regard as others expressed much the same opinion.