I've tried connecting a lead between the earth terminal on the 541 to the earth terminal on my power block and also to the 555's chassis, but this made little difference.

Strangely, using the 541 with an Arcam power amp gives a much lower volume of buzzing through the speakers than with the 555. I don't think it's the power amp though because the 555 plus an Arcam pre-amp produces no mains hum at all.

Jeremy