Im using an older line 6 spider 1 x12 set to a clean amp channel Bass 7 Mid 2 Treble 9. Play with your bass/treb/mid a bit and drive knobs. It will differ from amp to amp. I also am using his MXR distortion + pedal but its really only more of a booster pedal and it really dials in the highs really good. Oddly, the brit plexi models didn't sound anything close to Randy's tones through my amp and I spent quite awhile setting this up going through each amp/model till I got close to his sound. He's also using a few other pedals which you can look up online that I don't have which would probably help a lot. Check for his solo patch too I uploaded.