+1 on the e11 with the AKG 550's. I had some really pleasing results with this combo. They paired really well largely due to the 3 stage bass boost which for me elevated the bass to the presence I like on setting 1 or 2. I sold it when I upgraded to the e18 and find this a definited upgrade in SQ with all my other headphones amd iems but don't find the pairing as good with the AKG'S.
Lots of people say the E11 bass is muddy and overpowering. I think it is very good so long as you plug it into something that already sounds pretty good. Also use headphones that don't have a huge amount of bass in the first place. I am sure it could mess up the sound if people used an already bass heavy pair of headphones with the E11's bass boost. I like the way my X3 boost the bass fow when I use my K550s as portable headphones, but when I am at home, the difference with the E11's bass boost is very noticeable. It is cleaner and deeper in my opinion. This suits my K550s well but is even better with my AD700s.