JaceyDepends heavily on how much you like the HD600 and what type of sound you are after. The HD650/6XX is more laid back (stronger bass presence, somewhat rolled off treble), while the HD660S should have similar treble and detail retrieval, but with more bass extension. Many people who truly enjoy the HD600 sound signature find the HD650/6XX to be too laid back. The HD58X would be an OK budget option, but probably not a worthy "upgrade" from the HD600. HD660S would definitely be the most expensive route and there are very mixed reviews on them. I personally own the HD650 and HD660S and prefer the HD660S. That said, I was able to snag the HD660S for $375 shipped due to an open box sale.