As for the wire used, that will be ok, as it only enables the field produced by the board in operation to be absorbed so to speak to a close by return point. That large block of aluminum would act as an overgrown antenna, holding a low charge, especially from being any where near that speakers magnet, and the pcb being so close, this field would play havoc on near by components that were sensitive to it. Transistors primarily in general have an amazingly low static transfer rate, until energized or connected in a shielded manner. The manufacturers method is just to use a gnd plane for return. As long as the leads reach, and able to be installed, and are not too long, as this could bunch up between the board and sink, it will work fine. As for tinning the ring leads, would help to increase the transfer, tho not really needed. As for tinning to help then hold together better, that would help, however I have not seen if those leads (Oring) would be kept away from the other contacts on the power pcb when everything is assembled together.