Wiis have a preamp before their output. Try connecting your amp to the lines before they reach the preamp on the board (the definitive trimming guide has info on how to wire to the line level input) Also, I would try using shielded cables for the audio running to your speakers. You can get this kind of wire super easily by cutting apart an aux cable. Just make sure you connect the shield to ground at least on one side, otherwise it could make your issues worse.