If you bang in a ground rod, and connect your audio grounds to it, that’s usually a great noise reducer, the snag with it is when you get faults. Imagine, the old amp, with the screw on connectors for power, the ones you put on yourself. The live gets tugged and the ground wire, curled up inside because it didn’t have a pin to connect to, touches. The strings on your guitar go live. With your new ground , current flows, lots of it, and hopefully trips, breakers and fuses do their thing. The possible problems with adding grounds is quite small if done properly, but your audio solution has to be good enough for faults with everything in your home. The last thing you want is many amps of fault current getting to ground via an audio cable that you might be connected to.