RUNNING POWERED SPEAKERS FROM AN IMAC

Audiowoman,
I'm happy to hear you like the Audioquest Dragonfly.
Why don't you phone the place you purchased it from and let them advise you regarding your questions above? I'll bet they get those questions all the time.

I recommend you try Audirvana Plus playback software, made specifically for the Mac. You can try it for free for a couple of weeks. Learn how to use it to turn off other computer functions that compete with the music playback, and dedicate some RAM to buffer the music stream. This may reduce or eliminate the clicking and give you an overall better sound.

You will probably feed a cleaner data stream to your Dragonfly via USB with this, and even a Nirvana-decoded analog feed direct from the computer will sound better than iTunes on its own.

i bet your speakers aren't shielded. try moving them away from the imac.

and..... your cables should be shielded also...

and .. and  ... the  2017 imac lost the digital out through the headphone jack.