Do you play the MP3 through the Abacus' speaker system?

If not, you are definitely getting a change there...

What would be more telling is to play the .wav you get from the Abacus through the speakers you monitor your computer on, and compare THAT to the MP3 of the same file. We all tend to play louder than we generally like to listen at, that's another thing to take into account (louder usually sounds better!) so crank that MP3 up there (if your monitors can handle it.

Speaker systems for arrangers are usually ANYTHING but flat, so I'm not at all surprised that you get a different impression when actually PLAYING your Abacus than listening back on monitors. But, all in all, I would think that the sound system has got a LOT more to do with your impression of tone change, than using an MP3 encoder at 192kbps.