Bluebird,
You're right about the mb's... Sometimes it really doesn't make a difference and depends on how many voices there are to the mb's... However, the keyboards speakers themselves can make a world of difference... Even though the PSR-550/640/740 all have 6 watt speaker systems, the 640 and 740 had much better speakers.. Plus the tweeters on the 640 and 740 worked where as the ones on the PSR-550 are just for show and are inactive. If I remember the 640 had 8mbs and a few more voices than the 550, but the 640's voices were stronger due to the better quality speakers....

Squeak