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SSE - white noise at 16KHz

I built an SSE to run a pair of bookshelf speakers on my desk. The SSE has Tung-Sol EL34B, JJ GZ34, and EH 12AT7 tubes. It has been fine up until recently, but I now hear a not so subtle hum and noise centered around 16KHz.

The noise is giving me a headache and making me dizzy. Nothing else around the amp has changed, so I've no idea what's causing this. I also tried Electro-Harmonix EL34's and exactly the same noise at 16KHz. Any suggestions?
I had something similar by clicking a volume normalization checkbox on Volumio.

I find it a bit ironic that Volumio describes itself as "The free and open source audiophile music player designed to maximize audio quality" yet they include an option that significantly reduces audio quality. Any manipulation of the digital signal results in "less than bit-perfect", which is contrary to Volumio's entire existence - and it sounds like they actually made volume normalization even worse than it should be.

Having said all that, I think I am going to give Volumio a try, because I had not heard of it until now and I have been trying lots of different options lately. How's that for more irony? - or is it hypocrisy? Meh.