Louis,

If I were in your position, I would talk with owner of the music store and ask if you can A/B the various sound systems during a time when it's convenient to him. If he or she is agreeable, take the keyboard and vocal setup to the music store, then let your ears be the judge. Everyone has their own opinion of what sounds best for them--but it may not be what you're looking for. Additionally, take some good midi files with you, and allow them to play while you walk around the store in various positions and listen carefully to how they sound.

Play the same midi files through each system and do not do a lot of tuning. Set the EQs to a flat setting, and don't use any effects at all. This is important. This will tell what the system sounds like without lots of tweeking. Some sound good after lots of tuning, while others will not. The one that sounds good with a flat EQ will likely sound fantastic when tuned and tweeked.

Good Luck,

Gary