Fairly 'reliable' at around about 2/3 of the total nominal impedance rating for a driver in a cab.
If you measure DCR, the driver impedance in cab is going to be the next rounded off common figure on the larger side of your DCR.
Eg, DCR is 5.8ohm, '8 ohm' driver.
I don't know how well that works for OP's PA cabs with crossovers and compression drivers hooked up.