Leigh, If you play the Tyros 4’s stereo samples (like the concert grand), you'll definitely need "two" Bose units. Mixing Yamaha stereo sampled voices to mono causes phase (effects) cancellation and results in a very thin unsatisfactory result. That said, Korg and Roland samples don't seem to suffer from mixing to mono like Yamaha's stereo sampled voices do. That said, I really like the sound of the Bose, and recommend you get two compacts, not one.