I experienced much the same as you. I tried using a single Bose L1 with a Yamaha PSR-S900, and the sound wasn't quite up to my standards...I used two in stereo, and found it drastically improved the sound, and also improved the mid-range, which I had found rather weak when using only one (mono).

Using two speakers in stereo, regardless of brand/type, is a must with Yamaha arrangers, because (and it is a well known issue) they do not collapse well to mono.

And yes, Bose are very good for moderate volumes, but fall a bit short for the heavier stuff.

Ian