I just read your last few posts to see what gear you are going to be using, and I can help. If I were in your shoes, I would keep the Mojo, pick up a Cavalli Liquid Carbon used off the sale forum, and get the Eikon with a balanced cable.
For one, the Liquid Carbon is "about the size of a book" and can easily fit in a backpack, as you requested.
For two, Eikon + Liquid Carbon is a super synergistic combo, and LC was one of my favorite pairings with Eikon when I reviewed the it.
For three, Carbon + Mojo is another synergistic combo, and an often recommended pairing. They both perform WAY above their pricepoint.
Four, both headphones are great all rounders when it comes to classical, jazz and EDM. Atticus is more midbassy, while Eikon has more subbass focus, so they both have advantages on EDM, I kind of prefer the Eikon for it because the bass is so clean and extends so far down into the lowest, deepest rumbles. I also think it is the better classical headphone because it feels more spacious. I would go so far as to say it's one of the best closed headphones for classical period (if not the best. IMHO, YMMV). Jazz is a super diverse genre, so hard to say between the two... in many ways, it's a wash.
In my opinion this is a pretty easy solve: Mojo + Liquid Carbon + Eikon w/ balanced cable. You'll be riding a sonic gravy train with biscuit wheels, my friend.
Cheers!