In my case with the PSR-S950, I use a single cable from the L/R-mono output and go to the 1/4-inch jack of the Bose. No problems.

Brian, the reasoning behind NOT using the RCA plugs is impedance mismatch.

Cheers,

Gary cool
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)