I would say that best practice is to try and keep signal cables well away from power cables, and we always try and do this by using opposite sides of the rack for each, and yes Ernie you are right, it would be great if all equipment had the power plug on the same side!

That being said, even when we have been unable to distance power and signal cables, I cant ever remember an instance of where we have had mains induced hum. whether that's due to current equipment standards or just pure luck, I don't know!

I agree that turntable level signals may be more prone to this than others but in the instances where the turntables are remote from the amp we typically install a phono preamp next to the turntable and then send the normal line level output over a Cat cable using baluns to the rack amp. The rack amps don't normally have phono stage inputs anyway.