Experiments with $100,000+ Speakers

Interesting, I normally use hearing aids to be able to hear high frequencies, but when I turned my headphones loud enough (with an amplifier and without hearing aids), I could easily hear the 5hz and 15hz sound. It's really interesting because it's not a flat sound but sort of like a low spinning helicopter blade. I assume that's because of the short wave length because the 15hz sound was "spinning faster". Does that really mean I'm able to hear 5hz frequencies or could that just be my amplifier and headphones not correctly matching the frequencies? According to specification of my headphones (beyerdynamic DT-770 Pro 250 Ohm), its frequency range is starting at 5hz. But looking at my cheap amplifiers (Nobsound NS-08E) frequency range, it starts at 20hz. Does anyone have any idea if I was really listening to 5hz and 15hz sounds and was able to hear them properly or is my amplifier slightly pitching them up? The sounds were different, so I would assume it was pitching them both up by the same amount. The 20hz sound was also slightly different to the 15hz sound.