Ehhh, something sounds very wrong... the mini shouldn't load that supply that much, and it shouldn't be running that hot. Even with the PSU being plugged into the offboard reg holes, the onboard regs are still in circuit, with no easily removed component to isolate the amp channels from them (short of cutting traces). I would carefully go over the board, making sure that everything is in the right place and correctly oriented, and no solder bridges that you can see. You might also post some good pics of both sides of the board at sufficient resolution and maybe one of us might spot something.