I have exactly the same 2013 iMac + specs as yourself (Got it about 2 weeks ago) and no such issues here.... So i don't think it is a general model specific issue... but something regarding either your OS X setup or config.. .or a hardware issue relating to your specific Mac. I cannot think what though.. based on what you have described...and tried so far.
There isn't a means of shutting off Airplay btw... but try powering down all your other Airplay devices such as the Apple TVs you have and the Airport Expresses...and see if that makes any difference. I doubt it but you never know.
Any (new) news on the USB 2/3 design change?
Nothing of note: As has been spoken about before, the main issues with certain hardware not being fully compatible with the changes Apple made to the USB2/3 routines and ports, is down to poor design choices made by the 'cheaper' Devs.... Basically these Devs took shortcuts to save money and didn't follow Apple's hardware design guidelines... and these decisions that have now come back to bite these Dev's customers on the butt.... Mackie in particular were caught out with their devices forcing them to release updated and redesigned interfaces that followed Apple's hardware guidelines correctly (i.e. they gave them a way to be upgraded firmware and driver wise) but in the process they have left many customers who bought the first versions, "left to their own devices" (Pun not intended) with no hope of a fix........... Its also funny how many of the Midi devices that failed under the new routines, all failed because they were all basically copied/stolen from one design... and that one design was created without any reference to the design guidelines isues by Apple... which is why they all failed in the same way!!
If nothing else, many hardware Devs may now have learned the lesson of actually reading and following Apple's guidelines in regards to how to design hardware so it remains fully compatible with any OS X changes in the future.....Certainly the ones attending that 'meeting' had done so.... though based on how many software Devs ignored the "Update your plugins to 64bit now" messages from Apple three years ago.. maybe not.... Time will tell.
Sounds like a partial justification + a dash of truth.
Absolutely! That was the consensus of opinion at that meeting....
I've been a Logic fan since day one, Apple not so much
As you know.. you and I are on much the same page in regards to this......
Ten years ago while driving through the southern US (to get a blues experience) I saw many poor young musicians with terrible equipment (and no computers) who were focused on the heart of music, the hardware mattered little, the expression and execution were everthing.
I still think my best work was back in the late 70's when the equipment was whatever I could lay my hands on.. (and more often than not.. that equipment was, to use a highly technical term... a load of bollocks...) and the music itself and who I managed to play along with... was what it was all about rather than how I created it.... and if i was using the latest /greatest thing.... to do it with!