Isolating audio ground between devices
I am going to suggest that you want a differential connection from the preamp to the power amp.  This can be any of three techniques: 

1) audio coupling transformer 

2) differential line receiver such as THAT 12xx series THAT differential line receivers

3) if it is available, a differential input on your power amp.

The audio coupling transformer has the advantage that it is easy to implement.  Jensen makes nice transformers but they are expensive.  

My company makes small PC boards with a semi-custom transformer on it that sells for considerably less than the Jensen transformers.  Performance is not as good as the Jensen but is exceedingly good nonetheless.  Trinity Electronics transformer card

There are many different differential line receivers available.  My current favorites come from THAT Corporation but TI, Analog Devices, many others make pin-compatible parts.  It is important to match the input coupling capacitors and use precision resistors in the input stage.  Any mismatch causes poor common-mode rejection.

Some power amplifiers have inherent differential inputs, even though the amp may normally be used with only a single-ended input.  Again - it is important that the input network (resistors, coupling capacitors, HF rejection capacitors) are all matched so as to achieve the best common-mode rejection.

The reason for using a differential input to the power amplifier is to eliminate any hum or ripple on the negative supply rail.  Using a simple singled-ended connection as suggested by others can lead to excessive hum or noise in the amplifier.