How hot is too hot for car amplifier?

An amplifier is considered too hot if its temperature is above 160F (or 71.111°C). At this point, most amplifiers thermal protection relay will trip and shut the amp off. Also, if the amp is too hot to touch, it is considered too hot.

Should my amp get warm?

From what I remember Amps get usually as hot or hotter than the outside temperature even being on without playing music. The smaller they are the hotter they may get. And when not turned on, after a cold night and you check them, they are way colder than the outside temperature.

How do I know if I have a bad ground on my amp?

It can make your amp cut in and out or if the amp is restating. Whining or popping or any audible noise that isnt normal. Can cause burning or even a fire! So post some pics of the grounds and the distro block that is welded.