The Yamaha pianos do sound the worst in mono...so that means that the sample is at fault, and not the system. 99% of everything else I throw into the L1 is amazing. I tweaked a piano to sound pretty good in mono, but i mainly use EPs anyway, so it's never been much of an issue for me. I'm a Rhodes man .... not a piano player.
(Thanx again, Russ!!!!!!)