The MB500 pedal contains a switch and an LED which comes on when the Boost feature is engaged. Not sure how the older RB Boost switch pedal is implemented. Perhaps someone who has one can take a look and describe.
I do know the 400RB/800RB switch was a simple latching switch with no LED. I don't have MB500 schematics to know the circuit used for the footswitch or if it will work with the simple scheme of the 400RB/800RB. Sorry
Just call me B-String 2
GK Club #488 Big Cabs #175 Peavey Amps #92 50+ Club #44
"the MB500 footswitch has an LED and that LED interrupts the direct signal. Therefore when you close the switch it doesn't close nor open because the LED has no power. A simple on and off switch will work for the 400RB."
Bottom line is that the boost pedal switch that comes with the MB500 will not work with the 400RB(II). However, a boost pedal switch for the 400RB(II) is easy to make.