It's been quite some time since I last heard the 650. I thoroughly enjoy my 58X, but would possibly consider adding the 6XX to the stable. Wondering what the main differences are between the two. For what it's worth, I owned the 660s, and prefer the 58X. I appreciate the little bit of coloration the 58X provides in comparison. I found the low end a little too shy on the 660s, and I also found it little too clinical, bright and neutral.