I have my standard headphone testing tracks:

Generally: Aether - Illusion: Perfect for testing bass and dynamics.

Basically everything that Qumu makes, he/she produces very high quality gaming remixes, if you like that stuff.

Also "Theophany", same category, same incredible quality, different style.

Surprisingly also the soundtracks of a lot of Korean MMOs, incredible production quality: Blade&Soul(<-) ; LineAge / 2; Aion...

I had the Aeon Noire for a week and really wasn't sold.
The treble really hurt my ears on many songs, even with the "lvl 3" dampening inserts, even as a Beyerdynamic user.

Speaking of Beyerdynamic: I know this isn't a fair comparison, since it's open vs closed back, but my Dt 1990 completely smacks the Aeon 2 Noire in terms of imaging and resolution for less than, or around half the price.

You can try to get a hand on a different defective headphone and salvage a working driver from there. But be very careful to get the right model. Beyerdynamic has a lot of drivers. Old Dt770 Pro drivers for example are not the same as new ones, even if they have the same impedance.

The left headphone driver is probably dead then and repairing it would either be insanely difficult or impossible. I repaired a lot of Dt770s and 990s in the last years und while they are very repair-friendly, this case sounds like it's done.

Was it described as defective? If it arrived the way it is, you might be able to return it, but I obviously don't know where and under what circumstances you bought it.

Okay now this makes me sad.

The Dt990 Pro was the first headphone that got me into the hobby and I still think that this is one of the best value headphones that I ever owned...and I have a lot of them.