I really, thoroughly am impressed with the E17. I love it, and I love that I can plug other things into it just to get a better-than-average source/amping. I have 20+ pairs of headphones and IEMs and the E17 sees a lot of time. But if the HE-400 was my only pair of headphones I might be a little dissatisfied, having heard something better (the M&M and the LDMKIII). If I just got the E17 and only had that, never having heard the other things, I would be pleased for sure, though.
In my opinion, it is worth the extra money for the M&M, provided portability isn't an issue. The M&M are tiny, adorably so, but there's still a wall wart you have to plug in, and full-size RCA cables, etc.
Although you'll see a modest improvement over the e17, if you have other hp's that you'd like to be portable then I'd get the e17. If you don't need portable then the small schitt stack is the way to go.
Welcome to the club and enjoy!!
Just wanted to point out that I am on the 400th page of the he400 thread.
I'm just eager to get the best sound right off the bat. I have my E11 for portable if needed so I'm definitely leaning towards the magni/modi. Thanks. Happy to be part of this awesome/expensive club! lol