Bridging is can be tricky. Not all amps are bridgable. If they are, they would generally have a switch on them to allow this, like NAD units do.
Second, a bridged amp will treat a speaker as half it's nominal impedance. i.e, an eight ohm speaker will be treated as four ohms and a four ohm speaker would be reated as two ohms. And, many speakers drop below their nominal impedance at the low end.