Frank,
All of the models you mentioned allow you to search during playback. The Korg sequencer worked without a hitch after the upgrades, the Yamaha also works just fine. Your VA7 can search too - when the screen is back to normal it is a snap to search for a new tune. Of all them - the Ketron kb's may be the best playback because they use their best sounds as their GM soundset. This makes for more relistic sequences. I have no complaints about the way the Yamaha sounds during playback, and the Korg was fine too. Roland has an edge in chord sounds but is very weak in solo sounds (GM set)Try an X1 or SD1 if you can before you decide. Then you'll see what I mean. If midi files are very important to you - you should be tweaking and rerecording like crazy anyway. Don't go with stock arrangements if it's your main thing. Personalize it, and make it shimmer! They can ALL sound good with a little HUMAN element added.