It could be the MAX1631 power supply IC which in the Empeg, will reset at about 2 amps. It may act faster than the poly fuses on board, but it might also be what is tripping. What seems to be happening is that pressing the switch is shorting your 5 volt line to ground. The switch is normally not connected to 5 volts, but when depressed, it is connected through a 10K resistor network. The only way pressing the switch should cause such a fault would be if the resistor in the network became shorted for some reason.

With the Empeg off and the button fully depressed, measure the resistance between a grounded point in the Empeg and the 5 volt jumper. If you see a low reading of a few ohms, you have a problem. At this point you can release the button. If you find a problem, try measuring the resistance of each of the four resistors in the resistor network for the push buttons. Unfortunately, I believe these are behind the VFD next to the pad for the button LED resistor network. Again, if you find a low reading, you have a problem. Since you only mention one button causing this, only one of the four should have a low reading. If you find a faulty resistor element, try tracing it back to the push button. If it is indeed connected to the button giving you the problems, have the resistor replaced.

Hope this helps,
Stu