I'm not amp/electronics savvy, but I know (off the top of my head) my old 5150 combo had two 6L6s for 60 watts. The head had 120 watts w/ 4 6L6s vs. what you'd think is the standard 100 watts w/ the same tubes. I think my Zinky Mofo was either 50 or 60 w/ 2 6L6s. So, it's not uncommon... Or maybe the Rebel 20 is rated low.