Exceeding passive radiator xmax?

Xmax for a passive is more or less the same as Xlim for and "active". This is the physical limit of excursion. For an active you will hear the voice coil former hitting the back plate ... not that drastic with a passive, but you will not be able to move more air.

To get the most out of a passive radiator system you'll need move someting like 1.5 times more air for the passive than for the active.

Either chose a bigger passive or one active and 2 passive of the same size. Then you be able to run the active to the max before the passive sets the limit.