I'm thinking about getting a pair of hd600s, and since I don't have a headphone amp yet I would likely get a schiit stack to go along with them. I could probably get a used schiit stack version 1 for between $120 and $150, and a new schiit stack version 2 would cost me $200. This got me thinking about what the differences there are between the two versions. The only thing I can see that is different is that the version 2 magni has a gain switch, and the version 1 does not. Are there any other differences between them that would make getting the schiit stack version 2 worth another $50 to $80?

I dont have any experience with 1st version of the Magni/Modi but I have the Magni 2 and it works well with HD600,but TBH the HD600s arent that hard to drive.

My Astro MixAmp Pro drove it to earsplitting levels for gaming. 

I own both an original Schiit stack and a Schiit stack 2 (I bought the first for work, then bought the second to have one at home later), so I think I have a good handle on the differences.
 
Audio-wise, with my AH-D5000s, there's very little difference between the two. The Modi 2 technically offers a sonically better DAC, but for the music I listen to (mostly Spotify HQ streams and HQ mp3 rips, nothing on the extreme end of quality) I can't really hear the difference between the 2 DACs. The big difference between the Modi and Modi 2, for me, is the way host computers handle the USB connection. The Modi is a bit more foolproof in my opinion than the Modi 2, while my Modi 2 (connected to a Mac) sometimes loses connection and requires me to power cycle it. It's an extremely minor difference, but I have noticed it.
 
The Magni and Magni 2, for my purposes, are basically identical. They're both fantastic low-noise amps that work great with my AH-D5000s and occasional use with my Etymotic MC5s.
 
I guess what I'm saying is you'll be happy with either of them. If it were me, I'd probably just go for the Schiit stack 2 if it's only a $50 difference. You get a better DAC, and you get a warranty, and Schiit's customer service is fantastic - especially considering how cheap the Magni/Modi stack is.