Heat Sink is Positive ground, what does this mean?

So a floating heatsink is either above or below ground?

Power Mosfets or power transistors are isolated from the heatsink, any reason why?

A heatsink can effectively be in free space, not connected to ground at all. It's purpose is thermal, not electrical.

I've only worked on one amplifier that had a very wierd cooling system using heat pipes.

Mosfets are likely isolated from the heatsink for safety. I'm sure that you would not like the heat sink to be at say 300 VDC?

You don't have a choice as to what the tab of the MOSFET is connected to. The substrate of the MOSFET is bonded to the case/tab.

When you do thermal calculations, the mica or insulating washer has a thermal resistance . Eliminating the washer could improve the effectiveness, but only a small amount. See: https://en.wikipedia.org/wiki/Thermal_resistance

Here is an example of a circuit where you might want to have a heatsink at a voltage.
https://www.google.com/imgres?imgur...a=X&ei=Kt7QUeK5EcbpiwKPpoD4Ag&ved=0CDIQ9QEwAQ

Notice all the 2N3055 collectors are tied together. From the datasheet for the 3055 -
https://www.electro-tech-online.com/custompdfs/2013/06/CD00000895.pdf
You can see the case is the collector. So to isolate the heatsink you would need insulators under each transistor and wires to hook the collectors together.
So you can see if you were going to build a million of these and the boss said you could have all the money you saved you would have the heatsink at a voltage.