Look up the datasheet of the protection IC used. (µPC1237?) It probably has an "AC detect" function to shut off the output more or less immediately after the power has gone out, with a sensitivity (secondary voltage) adjustable over a wide range via a resistor.
We had this topic come up recently with a uPC1237 board (mod included). The problem there was that the OP wanted to switch to a SMPS, where there's obviously no mains AC as such. I recommended a small (3 VA max) additional transformer.
im new to forum. not sure i can post direct links to ebay. there is active item on british ebay now "Speaker protection board Power supply Rectifier filter DIY kit for amplifier". Also googles well with whats written on the board "integrated filter protection board".