The problem is not with the mono L1 Bose system...it is a well known issue with the Yamaha arranger, which has phase cancellation when using a single powered speaker system (mono), of any brand.

If I use one powered speaker, of any brand, that is plugged into the Yamaha's L+R (Mono) output, there will be phase cancellation, which in my opinion, as well as many others, is unacceptable.

Some people are able to happily live with that issue, but, in my experience as a Yamaha clinician, most cannot; the only real workaround is to use two powered speakers, or a stereo amp and two speakers.

I never recommend a single amp/speaker system (mono) with a Yamaha arranger, although, I'm sure there will be those who will. I am under strict orders from Yamaha to always use a stereo system when demoing.

It is not strictly a stereo vs. mono issue...it is about Yamaha's well founded phase cancellation phenomenon.

Ian