Even though I opted for the Studio series over the 600 series a couple of years ago, I don't see why you'd want to swap out the DM603 for the Studio 60. Reading between the lines, it seems like you've been presented with a great deal on a pair of the 60s, but can't hear them before you lay your cash out. Unless you know for sure that the Studio 60s will address whatever shortcomings you perceive with the 603s, I would not jump on the Studio 60s. And for all you know, the 60s might have their own set of characteristics that you dislike.

Anytime you do a speaker upgrade, you need to first decide whether or not to upgrade, and identify the reason why. THEN, you go about identifying the alternatives and which speakers meet your current criteria.