Linear power supplies voltage sag as they're overloaded. This famously exploited by some players to create "brown tone", or audio compression, as the input signal gets louder. I don't particularly care for this type of compression and a lot of other players don't either.
As far as safety, the input transformer is -boosting- your 120vac to 400vac-500vac :) So it's not protecting you from anything :D
My topology could be arguably safer... First, 120vac->24vdc via switching PSU. This actually provides the isolation you refer to. Then the 24vdc is split, part of it is switched down to 6.3vdc for the tube heaters and the other part is put through a ZVS boost circuit 24vdc->376vdc. Even with the double conversion, my efficiency is about 78%! I have complete isolation from AC via the first power supply. I bond my negatives to earth for safety. The best part is, this amplifier is dead quiet because the switching noise is very easy to filter.
