I feel it would be helpful to understand playing in stereo as applied to keyboards.

Two different signals amplified separately. Panning or degree of panning refers to how the instrument is has been set up. A trumpet for instance may be sounding from the left speaker and not the right, or it may be 40% on the right and 60% on the left or they 50% to each side. Normally the purpose for panning in a stereo mode is to capture the same sound you would hear if you were standing in front of the band. Stereo will make the sound more realistic. If you were standing on the left side of the speakers you would hear what you would hear if you were standing on the left side of the band. Stereo is sometimes used for a desire effect; it can give an effect of an instrument traveling from one speaker to the other, a moving effect.

Having said all that what happens if I do not wish to play in stereo or if I do not have a stereo amplifier as in the Bose compact amplifier? For this reason most keyboards have an R/L output. This output combines both sides of the stereo into one output, right and left sides are combined they are one.

When you use a Y wire to connect right and left to a (mono) amplified sub you are doing what the manufacturer has already done with their mono (R/L) output.

John C.