"...clean...thin.." The problem is not so much an amp situation as it is a speaker. When you mic a speaker it is pushing air, a physical reaction you cant really duplicate. However, I recommend a compressor with a slower attack and somewhat quick release. About 4:1 ratio. Then bring that input up and the output down until you see about 4-6 db reduction. That will give the guitar a fuller sound with some push up front, but without causing much if any distortion in your clean sound.