Hi Squeak,
I have a Triton Studio. I don't own a Triton Classic, but I heard that the Studio sounds better than the Classic.

I'm also lurking/posting regularly at the Triton forum. People there say that when you try to MIDI the Triton (studio) with Yamaha Motif Rack. But I'm not sure if this translates to the same problem if you're connecting with PSR550.