In the case of the Tyros4 and other Yamaha arrangers (S910/S900/3000) it isn't the fault of the mono Bose system...the latter does exactly as advertised, and is an excellent system for the single performer.

The fault (if you could call it that) lies with Yamaha's way of stereo sampling...using L/Mono out doesn't cut it for stereo sounds like piano, electric piano, or effects like delay and rotary speaker. If I plug my Tyros4's L/Mono into one of my Yamaha MS60S powered speakers, the sound is thin and one dimensional to my ears, no different than one Bose L1....with two speakers, of either brand, in stereo, it is a world of difference.

A friend of mine has a Kurz K2600 and it is always amped in stereo...he got the same results as you when he played it in mono...he's using a Traynor K4. http://www.traynoramps.com/products.asp?type=1&cat=57&id=340

We must realize, that Yamaha arrangers are primarily for home use, and 99% of users will plug their keyboard into their home stereo.

BTW a synth would have to go a long way in bettering those Kurzweil Strings.

Ian