Here's the latest:

1/4 inch input into Bose
* One unbalanced TS Mono cable out of the Korg Pa900 sounds great!
* Two unbalanced TS Mono cables connected to a Y adapter with a balanced TRS Stereo plug into the Bose loses the mic input into the Korg.
* Two unbalanced TS Mono cables connected to a Y adapter with an unbalanced TS Mono plug into the Bose (either the adapter or 6" cable version) sounds great. Jury is out on whether the sound is better than just one cable.

RCA Connectors
* I have tried 3 different pairs of cables (pair of unbalanced cables with RCA adapter at the end, and 2 different makes of unbalanced TS to RCA cables). In ALL cases I get a hum as I turn the volume on the Bose to 9 o'clock on (regardless of the volume level on the Korg, in fact even if the Korg is turned off).
* I have only the Korg and the Bose connected to the surge protector.
* The same thing happens if I connect my Kurzweil SP4 to the Bose.
* If I connect the keyboard to a Roland BA330 RCA connection there is no hum.
* If I connect my iPhone to the Bose there is no hum.
* Having talked to Bose and Sweetwater technical support they sent another unit out, which arrived yesterday. Same problem as the first unit.

1/8 Input into the Bose
* Have never tried that, but have ordered the cable Gary recommended.
* I have some other cables and adapters that will get me into that, so I'll try that later if I get time.

Bottom line, I am still happy with the Bose, but am just trying to get the best out of it. It seems like the RCA connection is designed for iPhone, but I'm going to call Bose again on Monday.