Can i bypass the builtin rectifier and directly supply dc to my microphone preamp and reverb board
There's no need to bypass anything; the rectifier will pass DC just fine.

The amplifier will operate at reduced power, mainly because the AC voltage measurement is an RMS measurement, which means that the peak voltage produced at the  output of the rectifier is more like &plusmn;16 V.

To get full power, you need a power supply that can put out a total of 34 V.