If you keep the volume and the lows down, you'll be alright. It's the speaker that you have to be careful with. I used my son's Roland Cube 20X before I bought a bass combo. The Cube is almost six years old and still in good shape. Here's a picture of my ‘woodshed’. The white P-Bass was still unmodified.
I used a guitar amp back when I was starting out too. It was a First Act, a tiny little box, twelve inches on a side with a five inch speaker, similar to the little one in the picture. I replaced it when I began to get the feeling it couldnt handle my five string.
