Hi Baz
I had a similar problem with my LS600. Sent amplifier to United Radio in New York America, no one in uk would look at the particular amp fitted to LS600.
Amp was tested and found to ok.
incidentally, amp is located in rear offside wing behind boot lining.
Fault eventually found In corroded wiring joint under rear seat. Check this joint, check all joints. It’s a great system when working properly.
Hopefully your car will be fine, but if it is an intermittent fault (ie corroded joint) the experience above could help. I’m assuming that at some time in my car’s past, a rear seat passenger may have spilt some liquid.
Enjoy the Ls600, it is one of the great motors.