For what it's worth, there are two different grounds in most of our products.
The safety ground usually goes to a lug near the IEC socket, which connects to the chassis. Transformer and choke cores go here, and sometimes a heater winding. The purpose is to drain hum that is capacitively coupled from the power line, to the safety ground. None of these are directly in the signal path or signal current loop.
Separately, there is a signal ground bus, usually attached to the chassis near the input circuitry. As much as possible, the routing is chosen so that any ground currents create the smallest possible signal at the sensitive input circuitry. The chassis connection point approximates a single-point ground - it usually has many wires coming to it or to very nearby ground lugs. These are the ground points and wires that have a significant possible effect on the sound.
The two are connected to each other through the chassis, since both attach to it a one point each. By separating them, and by chassis-grounding the signal ground near the input, any hum or noise currents in the chassis plate (i.e. between these grounds) should have minimal effect on the signal circuitry. At least that's the theory part!
