On the socket panel of my Beomaster 6500 there is a line in/out.  Can this be used for an external power amplifier?

I am not sure what socket you are talking about, on my 6500 I do not have a line in/out as such, what I have is as below

He means the input/output marked equaliser. This is a line in and out socket. The problem with using this is that it will remove the power amp[lifier in the 6500 so you will lose your outputs. I don't know what happens to the power link connections.

The 5500 has a similar system but has RCA sockets connected by a pair of metal hoops. I actually run my headphone amplifier from this and then run the output back again. 

The other problem with this socket is because it's designed for a graphic equaliser it is spurred off the circuit before the volume and tone controls. This means that it is not volume controlled.

Very useful for a headphone amplifier though!

On my B&O 6500 I have a Fosgate Surround Sound Processor connected to the I&O equalizer . It has worked great for years ; volume control is easily adjusted by either the B&O or the Fosgate remotes .