That’s the cutoff frequency without load. Concerning the lowpass, the source resistor and load impedance act as in parallel, so the cutoff frequency will be a bit higher.
Hi,
I’m wondering if tgis design has been tested by someone and has been found safe to use?
Well, I’m still alive and uninjured 
I did figure out that the distortion above 75% volume is due to my amp, not the transformers, since it also occurs on a pure resistive load. Maybe it’s possible to go up to 100% on a higher supply voltage, but I definately don’t need that much power.
Thanks for sharing this nice project. One question, what kind of power supply did you used? Ali has a lot cheap switched supplies but, I don’t think its a good idea to use one?
I’m using a USB-C PD trigger board to get 12V from a power bank. This guarantees isolation from mains. I would not trust any cheap power supply from aliexpress.
These amplifiers usually take any voltage between 9V to 24V, so another option is to use a battery from a power tool.