I'm using an Aurender server and plugging it straight into the Mac USB port. Speakers are Harbeth Monitor 30.1s. Speaker cables are probably 10 feet each. I don't think it's an issue with cables because the same cables were used with both amplifiers and the sound is dramatically different.
To me (and I emphasize that this is MY opinion, and I'm not trying to stir up a well-worn debate or hurt anyone's feelings), the Mac sounds unnaturally warm and flat. I notice a real loss of subtle detail and "air" to use an overused and silly audiophile term. I find myself turning up the volume thinking that the missing details are there, just at a lower level. But they are not. It's as if the information is somehow glossed over, perhaps by the autoformer output stage. In short, the musical excitement is not the same for me.
But to be fair, a lot of people prefer the warm, smooth sound that Macs are known to deliver and that's 100% okay. I suppose it's akin to a romantic tube amp. I'm just finding that it doesn't suit my listening preferences. The Mac sounds obviously colored to me, and I don't get the same emotional involvement with my music that I had with the Hegel. But thanks to all on this forum for your suggestions!