I think kbrkr is right. I too would play left hand bass on my boards with 61 keys. It got too crowded. There wasn't enough room for both left hand and solo work. 76 keys really changes that.

Squeak
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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.