Bias the OP valves to cut off (Blackstar) or simply short the speaker feed but I don't know of any mnfctr that does that.
Dave.
Not to go off-topic here...but you might find this interesting.
Right now I'm using some high-end amps from Carol Ann Amps, and their more recent build models now all come with an auto-bias system.
This is something no other amp manufacture has ever done, AFIK...it's a completely original design by Alan Phillips.
Basically...you can bias the tubes with a single button press, and it also monitors the electronics and performance, so it acts as a front line protection scheme that prevents any major amp failure. It will power off the amp if any faults are detected...and it will also warn you of a power tube going bad or one that won't bias. There's multi-colored LEDs that signify different states.
He also incorporates into that design an easy way to exchange power tube types...by way of the auto-bias system and a couple of dip switches internally.
I know other amps that allow the use of different power tubes....I owned a TDH Flexi for awhile that could do that, and it also provided a couple of external bias points, but you adjusted those manually with trim pots...but I don't think any other amp manufacture does a microprocessor controlled tube power amp.
You can read about it here:
https://www.carolannamps.com/uploads/2/7/3/6/27361629/carol-ann_autobiasmon_v1.00.pdf
He does a video demo of the system here:
Carol-Ann Amplifiers News
I've got three of his amps now...an early model OD2r that is cathode biased, so no auto-bias system needed...a Texas 50 that has the earlier semi-manual BiasMon system, and most recently a Revo 1 MKII that has the full AutoBiasMon system.
Anyway...this guy builds absolutely phenomenal guitar tube amps. I have one more of his amps in sight...for maybe later this year.
Everything is over-built...superb circuit design and implementation (the guy has an engineering degree...he's not just someone who builds by cloning)...and he's also a player, so he knows how to tune each individual amp build to get the best tones. He doesn't just follow a set blueprint and assembly-line process. Each amp is given a one-off kind of build that he adjusts as needed.
When I got my Revo...I liked the amp, but wanted a couple of tonal tweaks that departed a bit from the original design and to suit my tastes more...so I sent it to him, and he returned it to me with every tweak like I imagined it. He knows WTF he's doing.