I picked up a cool GK 400B Head (not 400RB). Early 80s I think. It is about 280 watts, has two 4 ohm outs, loud as fugg, and has whats labeled as a "Pre Amp Out" and "Power Amp In" on the rear. Is this the same as an effects send and return? Or is it a DI out? Or neither? Any help is appreciated.