Never played a Hiwatt. I've played a pair of OG JMP 1959s but they were MV modded. That said I did crank them and didn't find them any louder than any other good 100W amp. IMO Marshalls have the perception of insane loudness compared to other 100W amps mostly from where the upper mids sit (in a good place) and how the human ear reacts to these mids, not as much from actual measured decibles. This is why a 100W Marshall may be able to out-compete a 150W Mesa in a live mix- it's the EQ, not the power / volume. Unlike our ears, The decibel meter I've found to be less concerned about EQ.
IME the SLO100 and Wizards are abnormally POWERFUL for a 100W amp. They move air like a 150W amp, which takes power to do. Similar to why bass amps have to be many hundreds of watts.
