FX Loop Mod on my 3 channel dual rectifier

I hope I don't get skewered for this question because I'm well aware care must be taken anytime working inside chassis of any amp, however is it necessary to drain voltage to perform this relatively simple mod?

Yes. If you unplug it and leave it for some time, the caps should drain automatically. Just check the power supply with a DMM before you begin.

That will work, but only for the power supply.
What will be quick rather than relying on bleed resistors, after you unplug the amp, remove the power cord, then turn off the standby switch to connect the preamp and power amp to the power supply this will also help discharge the energy stored in the large caps on the PS just in case there is an issue with the bleed resistors. When you remove the chassis from the amp after giving it some time to discharge, you can check to see if there is voltage on the caps with a volt meter. Don't forget to put the amp back into standby so when you power it up again it will have a normal warmup. Also, just in case there is residual charge on one of the caps as the large ones are in series, one may seem to have low or no voltage but the other may still have potential. keep the switch in standby to minimize any shock hazard when working on the circuit board.

Before I work on a circuit with high voltage potential, I confirm it is safe before touching the PCB.