I am still waiting to see if someone will EVER truly prove or disprove this once and for all. There are several ways for an externally summed signal to change compared to the internally summed (gain structure, EQ's even if not engaged, output eq's, etc.). But I'd sure love to see a recording of the stereo out, summed to mono in a DAW compared to the same file summed to mono internally and recorded mono. (Only way to prove it, IMO)

Me, I just think the Yamaha pianos (especially) sum to mono very poorly, and it doesn't really matter much WHAT you play them on. Unless they are in stereo, they are going to sound pinched and phasey. You can possibly EQ a little externally to compensate a bit, but it is still there.

I haven't yet heard how loud the Compacts can go, and still stay clean (and not engage the built in limiter), but I HAVE pushed an L1 close to it's limits (pretty loud!). But all in all, unless you are doing live club work where you normally need a decent sized PA to get the floor pumping, I'd go for the two Compacts and preserve the stereo. Dealing with how much the sound changes in mono seems to be the determining factor. If Yamaha's sounds collapsed to mono more gracefully, it would be a tougher call, but those things are DESIGNED to work in stereo. Why hobble them?