While listening to music late at night, I noticed some sort of voice radio crosstalk coming out of the speakers very quietly. I put it down most likely to the length of the shielded cable between the Macbook and the power amp -- about 5m, of fairly shitty cable. So I changed the cable for a shorter, heavier-shielded one... no change, still crosstalk. So I plugged the cable in to another source -- an iPhone, as it goes. No crosstalk! Which means, definitively, that the radio interference/crosstalk is coming from within the Macbook.
You shouldn't use a headphone line/output from a laptop as an audiophile source, but even so, radio crosstalk is BAD.