Yep, a 100 watt Hiwatt with a guitar is obscenely loud and clean as a whistle. I don’t think it’s sterile and lifeless but almost painfully “in your face”. It was a different time; the name of the game in the olden days was to design amps to be as accurate as possible, and Mr. Reeves certainly accomplished that mission.
Remember, the “improvement” between tweed and blackface Fenders was less distortion.
That pursuit of loud and clean is also a Sunn trait, just one more of the many similarities between Sunn and Hiwatt IMO. However there are definite plusses to the loud and clean. Both amps take pedals extremely well, so it’s pretty easy to get dirty with an add-on. There are always ways of “dirtying” an amp up, but there is no pedal that will clean up a nasty amp, so again IMO the “big clean” amps have an inherent advantage overall.
This super clean Hiwatt characteristic was address by Reeves when he designed Pete Townshend’s signature amp, the CP103, which uses additional and linked gain stages and a slightly “hotter” output to fatten it up a little, but it’s still a clean machine.
It seemed to work pretty well at Leeds though…