I'd say attenuator. Followed by less and/or lighter speakers that would be ridden hard even with the decreased output of the attenuator.

It would be amazing to have both ranges footswitchable AND simultaneously available in reasonable SPL country. That would make a great gigging amp.

Fact is...that even at most club gigs, hell even arena stages...stage volumes are way down from where they were even 10 years ago. Mostly a good thing (and why Pete Townsend can play onstage anymore)
Funnily enough....the only times I get to really turn the wick up thesedays is in the studio.
Backwards land, man.

Lots of clean headroom is one of the points of Hiwatt sound, not the only one.
What about that amazing Pete Townsend clear overdrive and SHRANNNG!!??
Lot of great overdriven sounds came from Hiwatts historically and cannot be duplicated well by other amps: Martin Barre with Tull...The Stones...Tommy Bolin, Rush, Joe Walsh, Fripp, even Jimmy Page.
These classic and distinctive tones can't really be done well with a Marshall, Vox, or Fender, and it is hard to get into the crunchy part of Hiwatts without scortched-earth volumes.

I personally would buy a HeyWhat that could get into those dirty regions without sacrificing the classic clean or triggering a visit from Homeland Security.
Maybe several.