I have a Line6 spider 212 guitar amp. 2- 50 watt speakers, i believe. Its 100 watts total, i know that. I have my computer plugged in to the normal port through a stereo to mono adapter. How much would it take to mess up something on the amp?
The only thing that should mess up anything on you guitar amp, is a short-out or power-surge. VERY RARE. It should be perfectly fine to run music through it, its just the same as using a guitar on it. However, now you are using vocals and vocals are very high pitch, meaning the amps woofer has to move more for the vocals, which can overheat it and melt the woofer. I would call the company it’s made by and ask it that will hurt it, just to make sure it will or won’t.