Hmm. Slash's sound is actually not very special. The way that he play is though, it's hard to SOUND like him. (if you get what i mean) Now i don't really know what the particular amp you have sounds like. But to get Slash's sound, you need a good mid range frequenzy. So if there is an EQ on the amp, turn up the mid. For the gain i would suggest you, that you find the one that sounds most crunchy with most bite.
I think it's hard to replicate his sound with ''not to expensive'' gear. But i aint saying it can't be done :)

Good luck