Actually just for anyone reading this in future I'm not sure now if that combo would work well as Sennheiser only rate the gsx1000 and gsx1200 to 150 ohm for headphones..
Any other suggestions on a good gaming orientated amp or alternatively other good quality, great sounding headphones that have a 150 ohm rating that people could recommend?
Edit: Did a quick search and this looks alright.. Creative Sound BlasterX G6 Hi-Res Gaming DAC and USB Sound Card.. rated up to 600 ohm. I'm only going to get the 250 ohm 990s so I think this should work.. any opinions on the BlasterX G6?