It might be a good place to start but likely not end there. Many variables at play here: room; sub position; steepness of frequency roll off or cut off in the filters; etc. Chances are, with everything set at 80Hz, everything will play somewhat above and below that setting. For example, one time I had an 80Hz low-pass filter on my mains and ended up setting my subs at 55Hz for smoothest transition in my room. For me it was faster and easier to dial in my subs with an SPL meter and sub setup CD than by ear.
