i have a Focusrite 2i2.  A few years ago, i sat down with a salesman to audition headphones for it.  i called ahead before the session to let him know what phones interested me (an expensive mixing pair), and he told me the 2i2 didn't have strong enough drivers to run them.  He eventually sold me a pair of Focal Spirit Professionals, which sounded fantastic--to me, at least.  They were half the price of the pair i first mentioned, too, so he's an excellent salesman.  Anyway, i wouldn't have suspected the same issue with a larger model.  The problem seems to run in the family, i guess.

i'm not the most experienced guy for this opinion, but i have to concur.  i also have to apologize for not chiming in sooner.  i own the Presonus of your original posts and have had success with it--for my limited projects, anyway.  It came packaged with a plugin host i bought, and the company sent the Presonus with one of these.  i've used it to plug the Presonus into my computer when i want to record and have never had an issue.  The MIDI connections even show up in Patchage.  Again, my apologies for not chiming in sooner.  i don't own an iPad and didn't think i could help with that issue.

Keep trying Ralf, though i won't blame you for not trying the Presonus, again.  You'll come across a solution, soon.

Tom