installing a low wattage speaker would be a great way to learn how to replace speakers. You need a low SPL speaker, not wattage. Two different things.
I don't think most of you in this thread realize how loud just a few watts are. I've owned 1/2 watt amps that cranked into a 4x12 could be heard in my driveway with my doors shut.
It's a master volume amp, I wouldn't worry about it. If you get very much power amp distortion going from it being cranked, you're just going to add mush to your sound. I'd get the RM20 because it's cheaper, small, and sounds great. I really doubt in a bedroom/home situation you're ever going to notice the difference between 20 and 80 watts. Maybe the tube TYPES, but not wattage.
Pete