Its not so much how high the sub will play as it is at what volume. I have modeled several 10" drivers where extending the tuning of the box down to 20Hz reduced the output higher up. In two cases the driver started to see reduced spl starting at 80Hz.
Taking a capable 18" driver with a lower tuning, it showed no roll off until almost 1000 Hz. There is no replacement for displacement.
Even with an 80 Hz crossover, your sub still needs good output beyond that to blend well with the mains and keep up without distorting.
