Yes, the stereo expander really makes the Motion Sound amps... they'd sound like two stereo speakers turned back-to-back without it. The expander is just a phase inverter like the kind used by so-called "vocal eliminators" so it does have the side effect of reducing the volume of instruments that are in the center of the mix and enhancing the reverb overall, but I find this to be a pleasing effect that tends to act a little like compression and give a little more "blend" to the sound. The clarity is there as well as power to spare, all in a very portable cabinet that weighes 55 pounds. And yes, the sound does seem to come from all around the amplifier - the effect carries over into the room also. You can't really imagine it until you hear it.
I was using (and still have) various PA components for amplifying my keys and they all work fine too, but Motion Sound put everything I need in one compact box with several bonuses: there are stereo XLR direct outputs on the back of the KP-200s so I can simply plug my EONs in and either expand my sound field or create a surround-sound presentation, and there is a "click input" that does not get passed back out the XLR direct outputs which is perfect for taking a vocal monitor feed when playing with a band - so the KP-200s can act as both keyboard and vocal monitor. It also has a lo-z mic input with it's own controls, so at a recent corporate gig my KP-200s did double-duty as both my keyboard rig and the guest speaker's PA. It was in a mid-sized auditorium and the KP-200s was more than enough.
As long as the mind-set for stereo keyboard amplification is "use a PA" or "buy two" then there won't be much demand for innovation, which is why nearly all keyboard amps look alike right now. I wouldn't even bother owning a keyboard amp if the Motion Sound series hadn't come along. Like me, you really need to hear it to get it.
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Jim Eshleman