It's possible it is the adapter. If it's not getting enough juice it may not perform properly. Maybe those few extra volts are what the amp needs.

On a side note (GREAT KEYBOARD YOU GOT THERE). I've been missing mine A LOT lately.

Squeak
_________________________
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.