Nope, it's definitely not your imagination. It is a well known fact that Yamaha arrangers do not collapse well to mono. Yes, they will play in mono, but mostly all of the samples, the stereo effects (delays, chorus rotary spkr) and the voices/drums in the styles sound at their very best when played through a stereo system.

I've used Bose, and can vouch for your findings. I used a pair of Standard L1 (not the Compacts) with both a PSR-S910 and a Tyros3 for an entire summer of concert work, and the difference between using one (mono) and two (stereo) was quite substantial to my ears, and those of my listeners as well. A Bose system in stereo far outperforms any other stereo system I've used...not only is the coverage far superior, but the stereo separation is evident at a much further distance than a regular system.

If you sweat the details at all, like I do, you will prefer the Yamaha in stereo. Of course, if you're old and/or your hearing is shot, or, you can live with the sound being compromised to more than a fair degree, then using any speaker system in mono would probably be okay.

I experimented with one and two Bose, and after hearing the Bose in stereo, I could not go back to mono...everything sounded flat and one-dimensional in the latter, and, although I suppose one could force themselves to get used to it, perhaps to save money or shorten set-up/break-down time, I just couldn't live with it on any level.

Ian

PS...just as an added note...when I was demoing arrangers for Yamaha, they were explicit in their instructions that I use a stereo sound system, even if it meant using one from another brand.