IIRC, 80hz was picked as the THX spec frequency for no other reason than the engineers felt most home speaker systems could play down to that level with out problems.
As Bruno said, nobody says you have to set your crossover that high for speakers that can handle playing lower. I run my M80s at 40 hz or 60hz depending on my mood and media I am listening to. In my old house I had them set at 60hz 90% of the time but in the new place I can run 40hz much more often, it doesn't get boomy anymore as I now have a much bigger room
