In my opinion, an attenuator would take something away from the amp. Yes, it can be a loud amp, but the Herbert does a great job at even the lowest volumes. Herbert definitely has enough features to adjust the tone so that you can play at all levels and have it sound awesome.
I agree with fishbeiner,
I’ve tried the Rockcrusher and Hotplate.
Honestly the master volume robbed much less tone than either.
The main thing missing with a Herbert at low volumes is speaker excursion.
The amp itself sounds great.
I have a THD hotplate. I NEVER use it live or for recording but it means if I'm playing super quiet really late at night it gives me a bit more sensitivity at the bottom end of the volume knob. It does get it a bit fizzier too though, so I'd say you definitely don't need an attenuator unless you want to play right near sleeping people and it's so quite the volume knob jumps between inaudible and just a tiny bit loud.