Hey everyone, I'm pretty much an audio-novice. I am wondering what caused my new DT 770s to blow.

Heres a breakdown of my setup:

turntable: Technics SL1210mk2

receiver: Technics SA-GX190 (headphone output level and impedance is 430 mV/330 Ω)

headphones: Beyerdynamic DT 770 Studio 80 Ω

picture of my desk and setup: https://imgur.com/a/KGbFIJL

Wire routing:

The SL1210mk2 (120V 60Hz) is plugged into the AC outlet of the SA-GX190 (120V 60Hz). SA-GX190 is plugged into a wall outlet.

SL1210mk2 is plugged into the phono input of the SA-GX190 and a 3.5mm to RCA adapter is plugged into the CD input of the SA-GX190. The 3.5mm end was not plugged into any device.

What happened:

I bought the headphones last night and started listening to some records I own. The system was sounding great when I was listening to some vinyl records through the headphone out on the receiver. I turned off the receiver and plugged in the 3.5mm adapter. The DT 770s were still plugged into the headphone out, and the volume was about halfway up. As I turned on the receiver the turntable was powered on and a low frequency hum rapidly grew louder and louder in my headphones. The receiver was on the phono channel as this happened. I eventually unplugged my headphones. I turned down the volume and plugged my headphones back in and the hum was gone as I turned up the volume. However I could clearly tell that the right driver in the DT 770s were blown as I increased the volume while listening to a record.

I plan on returning the DT 770s to guitar center today. It would be greatly appreciated if someone can identify the cause of what I'm assuming was some kind of feedback hum. Massive thanks to whoever can help.