Dave,

It hissed even when there was nothing plugged into it and the Channel 1 gain was set at 10am and Channel 2 gain at zero.

I have been told using a stereo to mono Y cable can eventualy damage the output of my CD player.

Larry raised an intersing point, I think he is right, that the L/R sum output jack of the Yamaha digital pianos does not sound as good as running both the L and R out. It's a pity the Bose L1 cannot sum stereo inputs without phase problems.
rintincop