Starkeeper,
I have to disagree with you on that one.. Although the 550 sounds good, it only has 8mbs to the sound.. The PSR-740 has 12mbs.. That makes a pretty big difference in sound quality... Also the 740 has way better speakers and tweeters that actually work so naturally the sound is going to be more crisp and natural.. If I remember correctly the sweetflute was also on the 740.. From what I understand voices like the sweet flute and sweet trumpet were taken from the PSR-8000 and added to the 740's voice set. The 740 may be older but as for sound quality and features the 740 is a great deal better than the 550.. I've owned the 540 and now the 550.. I've played both the 640 and 740 numerous times, and even the 640's voices are more crisp than the 550 because of the speaker set up... Again every one has a different ear for sound....

Squeak