thunderbolt audio

Not knowing what Apogee tech has already suggested I'll hazard a guess.....

Create a new user account, log in as the new user, if everything works as expected you will know it's not a hardware problem.

Or.... did you update anything recently, like the operating system? Even an incremental update, to El Capitan, any updates at all? You do have your machine set to NOT auto update, correct?

Could still be a hardware problem with the iMacs Thunderbolt port.

Did the MacBook Pro upgrade to Sierra?

Generally speaking, if audio and recording is important to you, turn audio-update off, you don't want a computer company deciding when to update your working system, that's your job. There have been several past reported problems after updates. Especially if the system was updated with the audio hardware plugged in.

Sounds like Sierra is the culprit.

Try this... (doubt it will work)

Turn off the iMac, unplug the Ensemble from the Tbolt port, restart the imac let it run for a few minutes, shut off for a few then restart, after it settles plug the ensemble back in. If the ensemble was plugged in during an OS upgrade there's a slight chance some of the OS drivers were not completely installed.

The Apogee site does say that theEnsemble is compatible as long as you have installed the latest drivers:

http://www.apogeedigital.com/blog/macos-sierra-and-ios-10-compatibility

Give it a go......