I prefer the GS1000 to the RS1 or HD600 by a wide margin.
The RS1 are too bright for my tastes, although I don't have a lot of experience with them so I base this primarily on my listening sessions with them at various Head-Fi meets over the years. I'm told that they can be tamed in the right system, I just haven't heard that yet.
The HD600 are comparatively flat, distant, and uninvolving relative to either of those high end Grados. Yet, I can see where someone would suggest getting both the HD600 and RS1 over the GS1000. Not for me, but I can definitely see the argument for having both corners covered. I just happen to think that the GS1000 are enough better than either of those two that neither of them would be missed.
If it were changed to the HD650 and RS1, I might just take that combo over a pair of GS1000 because the HD650 really come to life with the right aftermarket cable (either the Moon Audio Silver Dragon or the Zu Mobius would do the trick for me). But then you're pretty much at the same price point as the GS1000 (or even beyond that if you buy the RS1 and HD650 in the new market, plus a high end Senn cable upgrade).
I also quite like donunus' idea of getting a pair of AD2000 and being done with it. To my ears, they bridge the gap nicely between the RS1 (too bright in the highs for me) and HD600 (too dull in terms of the distant soundstage). The AD2000 have some of the pep and pace of the RS1 although they don't come across as being quite so aggressive, which I think is a good thing. Yet, they're not totally laid back like the HD600. In fact, in some ways, they're not a lot unlike the GS1000, just not quite as "full" of a sound. Plus, they're a lot cheaper than the GS1000.
