Grounding two monoblock amps with common preamp

[GainFreak]

Hi,

I decided to upgrade my DIY stereo rack amp where I have two completely separate 2x6L6 amps and add a single channel preamp.
Currently the amps have their own power transformers and power supplies which are grounded to the chassis via resistor/cap/back to back diodes at their opposite sides of the chassis.
I want to add an option for simultaneous operation, single channel operation an as two separate amps. Since the preamp's ground will be their common ground point when ran simultaneously I was wondering whether I should leave the grounding as it is or lift the resistor/cap/diodes combo from one side. I tried both ways and the amps work without any hum or noise but I'm not sure which is the right way theoretically.
Also I'm adding a cabinet simulator running +/-15V wires to the opamps. Should I run ground wire from that PCB to the PSU's common ground or just ground it locally to the speaker socket's ground. Again it works both ways just interested what is the situation theoretically.

[Steve Conner]

The right way theoretically is to use balanced lines between the preamp and power amps. The right way in practice is whatever works without excessive hum. The resistor/cap/diode combos appear as open circuits to ground loop voltages, so you can have as many as you like. There is no need to remove any of them.

In op-amp circuitry, the power ground is also the signal ground. Signal ground integrity takes priority. Grounding the board to the speaker socket means that it has the correct ground reference for its input signal. But on the other hand, the input signal is so large that the choice of reference doesn't really matter. And if the speaker current flowed through a sizeable length of wire between the speaker socket and the system ground, then this could wiggle the cabinet simulator board's ground about and create a ground loop with whatever equipment you plugged into its output.

[GainFreak]

Thanks for the input. Actually the cabinet simulator output is transformer isolated balanced output with ground lift switch. Already tested it with my PC and the switch eliminates the hum.