You just need to make sure your subs do not go over Xmax at the operating power, but since your amplifier exceeds by far the RMS of your subs you will have to be very careful not going too far, generally oversized enclosures and sub overpowering in a hazardous mix with a very likely outcome of subs mechanical damage so you need to set the gain of the amp not to the clipping point but rather lesser that the subs can can tolerate.
You can find maximum power in a similar way as before, in WinISD up power until Xmax is reached in the enclosure size you chose and you can clamp the amp to roughly reach that power and set gains accordingly so you can't go over the limit on normal use.