Speaker Protection help

[The Dude]

Teach them better micing technique and they won't have to turn up so loud.

[J M Fahey]

Rereading the OP, he mentions customers bringing a POD or something plus "a Power Amp".
Presumably a rackmounted PA type.
I had been thinking about protecting the cabinet from "a 4 or 6 EL34" head, up to and including a Triple Rectifier or similar one, but a PA unit can have any power , afaik it might even be a 9 or 20KW Labgruppen or similar unit.
There's no amount of sensible passive attenuation or speaker over rating which can handle that, and on the other side, it will unjustly degrade "normal" 50/100 W tube heads.

So I start again from square 1: if we have a standard Marshall 1960 300W cabinet, (4xG12T75), which consists basically of 2 2x12" speakers sharing a box , and those 2 sub-arrays can be connected in series or parallel or used in stereo, what we basically have to protect is an 8 ohms, 150W sub module.
This can be achieved by an ***internal*** fuse in series with each speaker pair of about 3A .
Standard blow, not "T" or Slo Blo .
That cabinet will stand an up to 100W head, full tilt, all day long.
The fuse should be internal, "automotive" type floating holders, added at the backplate terminals, so both the customers can't replace or bypass them, and house staff can relatively easy replace them by unmounting the backplate (easier than pulling the full back panel).

The House might charge, say, extra $100 for *each* cabinet found "mute" at the end of the rehearsal.

Or they might switch to a safer, more relaxed lifestyle, such as driving fuel trucks across Afghanistan or teaching Alaskan Bears to dance Tango.