I just finished doing a search for the most likely culprits, and because you are using the RCA plug input, there is a possibility there is an impedance mismatch, which could also cause this type of problem. I don't know what the output impedance of your keyboard is, so you may have to contact the manufacturer to get the information. Then I would check with Bose customer support to determine the input impedance specs for the L1 Compact's RCA connection.

All the best,

Gary cool
_________________________
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.)