50 hz current has that awful effect in some cases - and will (very slightly) de-rate the amp. Given that those appear to be toroidal transformers, there is no easy cure other than an isolation transformer at the head-end. And that will be a pretty massive item, 20 A (@ 120 VAC) anyway.
As it happens, if that amp is still set to US voltage (120 VAC), there are some very good 240:120 isos out there that will do nicely.
http://www.farnell.com/datasheets/22...oaAlPcEALw_wcB
Nor is there anything wrong with an iso on the system for general noise reduction.
On the other hand, I suggest that you not panic, but see how it goes in your environment.
Peter Wieck
Melrose Park, PA