You'll probably be fine with the gsx 1000. The impedance recommendations don't mean much in practice, because most people aren't listening to these really quiet recordings that require a lot of amplification.
If you buy a new dac, then you won't be able to use the virtual 7.1 on the gsx. If you want an amp, you could do the jds labs atom or the schiit magni 3, but I don't think you even have to do that. When you get the 660s, I think you'll see that you won't be having volume issues with the gsx.
