Originally Posted By: ianmcnll
it is about Yamaha's well founded phase cancellation phenomenon.
I agree that the problem lies with Yamaha's unique 'phase cancellation' issue, and not with the Bose L1 or any other mono output speaker units used.
On the other hand, 'mono' samples aren't affected by this effects phase cancellation problem.

I still always insist on going out in stereo whenever possible,
but I've found (but only after upgrading to a Tyros 2 generation arranger and later) going out to a single speaker,
at least tolerably acceptable when going out mono is the only setup option available,
but only if you restrict yourself to playing 'mono sampled' voices.
Here's a thread I posted on this topic back in July 2006:

Tyros2 sounds fine thru a SINGLE mono PA Speaker now!

For those interested, here are some interesting links on the topic of stereo phase cancellation:
Mono Compatibility and Phase Cancellation
Avoiding Phase Cancellation
Mono Compatibility - How Important is It?