On the classic/vintage Rat pedal you have 3 knobs "Distortion" "Filter" and "Volume" If you turn the Distortion up and roll the Filter down (plus also play with the tone knobs on your guitar too) you can get some amazing tones out of a Rat pedal. You can control the amout of feedback/saturation by a couple of factors. Knowing what kind of pick-ups you have in your guitar (single coil or humbuckers) and the pick-up selection position. In the forward (neck) position you will get a richer bassier tone, and in the back (bridge) position you will get brighter tone. If you roll your tone knob back to compensate for the freqeuncies that might cause squeal on your guitar, you can control the sound of the pedal. Every guitar/pedal combination has it's own characteristics, so you need to play around with what works for you and "dial things in" to your own tastes.
BTW I have an 80s original version (which has been beaten up a bit, but still sounds good) and an 90's Vintage version of the Rat and I like the sound of both of them. Each has their own specific sound and combinations with certain guitars I use.
I hope this helps.