MacedonianHero said that the
GS-X mk2 was worth the upgrade. The difference between the GS-1 and GS-X mk2 not just running balanced, but the
boards were updated to provide more power / voltage swing, and there is obviously an upgraded power supply. While the tonal balance between the GS-1 and the GS-X mk2 is similar, the GS-X mk2 is a real upgrade. I had both in my house for a couple of months trying to convince myself I could be happy with the GS-1. I would say the GS-1 is like looking through a clear glass window (which is pretty darn good) and the GS-X mk2 is like there is nothing between me and the music. Of the amplifiers I have heard, nothing other than a few of EC tube amps have made the HD800s sound this good.
My life has changed in the last year. I am using speakers mostly, headphones rarely. I think about selling my GS-X mk2 (I get money and the amp gets used with the regularity is deserves), but each time I say to myself "I am going to sell it" I take an hour or so to A/B the GS-X mk2 against what I would use once it's gone and I find myself thinking "Ummm... I just don't think I can let go of the GS-X, I enjoy it too much, even if it isn't everyday".
Regarding the earlier comment about the Hugo. The Hugo is the best portable system I have heard when it comes to driving the HD800, and it's equal to several desktop systems I have listened to, but
as I wrote earlier, the amplifier in it doesn't compare to the GS-X mk2 (or other top amplifiers) drive from a high quality DAC.
--Mark