I'm lucky in that I can appreciate and immerse myself in many different sound signatures. I love my favorites equally. My wallet is of course not so lucky.
The bass on the 400's seem deeper to me, more full. But the presentation of the whole package, while not neutral, is pretty darn good on the 600's, and I can live without what I think is slight boost in the low mids on the 400's. I'd have to see a graph of the frequency response taken with my own ears to verify what I believe I hear.
In any case, I have two models that I respect equally. Must be a result of loving Grado's: many flavors of awesome. Except for the RS1, which I consider to be the biggest waste of money I can think of when compared to price and performance of the SR series. I mention this only to show that I don't love EVERYTHING...