The headphone out is designed with enough power to drive a small speaker (your headphones) and drive 'em loud enough to be heard in a club environment. It's also designed to be driving a low impedance.
Line level is a much lower level, and is designed to drive another piece of electronics.
Line level isn't a firmly defined level.
Typically for home electronics using an RCA plug, line level is around -10dBm and is a high impedance.
Line level on pro equipment, using a 1/4 inch or XLR connector is often 0dBm, and may be a high impedance (usually the 1/4 inch) or 600 Ohm (usually XLR)
Odds are pretty good that your headphone out would overdrive the input to your amp, and sound nasty.
I wrapped a newspaper ’round my head
So I looked like I was deep