The metallic ring is, most likely, caused by the post-ringing caused by the output stage, especially if using thin & longer headphones cables. Everything should be measurable, we just need to find the right approach. I will do some tests with several cans and see if I find one having pre or post ringing on FUN. Also, if THD increases a lot after 10 KHz, then the bright sound appears on the headphones; this is caused by the added harmonics and subharmonics. If THD is flat across the entire spectrum, then a more neutral and balanced sound could be obtained. Back to the subject, there's no noise I can get with my sensitive cans, unless I max out the volume (which is crazy). I was unable to find over-bright or metallic ringing on the headphones tested on this amp. I'll try some more cans and get back if news will appear.