In my continuous fight agains current/digital noise, another simple but great tip:

A silicon rectifier, be it a bridge or a discrete one, introduces noise in the power connections inside the amplifier circuitry. This could be almost suppressed by adding resistors between the power transformer and the rectifier.
Most amplifiers, power, integreate or preamps, nowadays use a balanced power supply. By inserting a simple resistor between the transformer and the bridge/rectifier this noise will be attenuated to a very low level.
I have done this in my old Rotel integrated amp with a 0,2 Ohm resistor and the sound became wider and deeper than before.