I truly did find an increase in the mids when using two Compacts, and also two Bose L1's.
However, it may not be enough for your tastes, John, which is why I suggested you try two, before you invest in a second one.
As Gary said above, the EQ on the keyboard can make a big difference in the overall sound, so I would try that first.
In my experience, however, two Bose worked quite well for my needs and my expectations of what I wanted to hear coming out of the speakers, plus it had the added advantage of superior stereo sound, especially if using a Yamaha arranger (they don't collapse well to mono-it's a well known fault).
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.