The empeg uses switch-mode PSUs - these aren't linear regulators and so they are much more efficient and don't generate a lot of heat (the mk2 runs even cooler with higher-frequency sw-mode supplies). It's also reverse-voltage protected, though this will dump current through the resettable fuse so you would get a nasty smell but no permanent damage.

18v does give slightly cleaner sound on the mk1. The mk2's PSUs aren't affected by voltage changes like this.

Hugo