Yeah, the S7 is a good choice if your preferred headphones are Sennheisers, but it wouldn't be on my list for an HE6. Try the S7 with a HD600 and HD800. It's a good late-night listening pairing based on the tubes I've heard, especially with vocals.
The first impressions others have written about the Mjolnir seem pretty positive, so it would be interesting to see how close it is to the GS-X at a fraction of the price. I'm not too familiar with Schiit gear, but it seems like people like their products a lot. I'm hoping to listen to a Bifrost.
If you liked the J2 with the HE6, it gets even better with a quality preamp. Mellows out a bit of the hardness without taking away the detail. I don't recommend DAC volume control. I'm hoping to give a 4-pin terminated HD800 another shot with my current J2 chain someday, as well as a LCD3.
Luckily the J2 doesn't have that many sharp edges, so I haven't injured myself on it
Reno Hifi has good deals on used and demo J2 units, and they do a full quality inspection and offer a renewed warranty. It's a great amp to have around when graduating to speakers. I have a feeling the J2 will outlive all my other gear.