The original RV50 had a 6V6 power section and it sounded quite a bit different than the 100. The 50 was looser and fuzzier, the 100 was tighter and meaner. To me, the Mk IIs were pretty similar between the 50 and 100w. I liked the 50 better because it had less headroom and was easier to get the clean channel to drive. I thought the 50w was better for metal in the Mk II, but the 100w Mk II wasn't as aggressive as the original. Haven't tried the Mk III.