So here is the manual I bought in 1979 for my first PL, a 700. It suggests installing 8 amp fuses for "Industrial or PA use" I now see a spec for power into 4 ohms.
About Bridging, I am driving the amplifiers with a miniDSP 4 x10 HD. The DSP functions as pre amp, EQ, and Crossover. It has unbalanced RCA as well as balanced outputs with up to 8 V RMS. Could I connect two series resistors (maybe 1 k ohms each?) across the +and - balanced output, then ground the centre point to the PL input ground. The + side of the resitor string would drive one channel, the - side would drive the other.
Voila! Amplifier is bridged! Am I missing something?