Hi Simon. After my last post above I did check directly with Hegel and they said they are completely happy with MBPs streaming and use them themselves in demos without need for any other product. I did try it with my Mac mini as well and the result was the same. I suppose I could try it with the MBP unplugged to eliminate the power supply to it but if it is an electrical supply issue the amp might be suffering from it as well.
It's like a faint cracking in the tweeters ...
It's like a faint cracking in the tweeters ...