hmm, depends what kind of balanced amp you heard the hd650 out of. while the lavry probably wasnt the best source, for a showroom type listen, the balanced outputs of it should have been adequate. it really depends on which amp you use with them i find. without a doubt, i agree with whats been said here, the 650 are a MUCH "pickier" headphone. I found that they did improve much over time, initially i was not that impressed with them either as opposed to the hd600s. if you like the hd600s, id say stick with them, despite there being obviously some major differences between the two phones, they both stick well within the sennheiser house sound that we all love so much, so dont waste a ton of money upgrading to the 650s if your unsure about it. I especially wouldnt bother doing it UNTIL you have an amp and source you feel might be able to get the best out of the 650s, until then, stick with the 600's which it sounds like you are more than happy with!
even still, if you ever DO get to the point where you feel like you might have an amp that can bring out the best in the 650's , try someone elses pair before you waste a ton on it just to find out you still prefer the 600s (as ultimately, as is with most things in the headphone world, its purely preference, i know many people who still prefer the hd600 to the 650, even balanced and out of a good amp). me personally, i just prefer that extra amount of refined low end that the 650s have, and after a good burn in, i dont feel that any mid or highs are sacrificed.