I am going to assume those drivers were in a ported enclosure to extend the output at the tuning frequency. If you can find out what KRK was targeting for the port tuning that will give you an idea where your limits are. If you can nab the driver spec sheet look for an Fs speck that will also give you a good indication of how low is reasonable. Keep in mind given the change form an enclosure to essentially free air in the door will also impact the performance and power handling.