Bass guitar into a guitar amp?

My guitar teachers always tells me that its perfectly fine to do this, but i have heard otherwise. Some people say that the low frequencies will fry the input/output and the amp will get damaged. I have also heard that only the speaker will be damaged, which seems more likely. Would it make any difference if it were a tube or solid state amp? Im interested in learning some bass, but I want to know if i would need a whole new amp, since i already have an expensive guitar combo amp (fender hot rod, iin caseit makes a difference to know what amp i would be using)