Re: HOLY DECIBEL . . . an AC30 is (((LOUDER))) than a 100W Plexi !
Lot of things to consider here, in my opinion.
First, many people confusses amp's wattage with the volume that their speakers produce.
Amp's watts are the energy that the amp has available to be transferred to the speakers.
What determines the final volume is the speaker's efficiency (or how much energy coming from the amp is transduced as sound preasure).
The Blue Celestions that AC30 mounted were one of the highest efficient speakers and, yes, the AC30 is painfull loud (with those speakers!).
We can talk about the load also, if your amp can work with a max impedance of 4 Ohm and you are mounting a cab that gives 16 Ohm you are dividing by 4 the watts. Try that 1959 with a single 4 Ohm high efficiency (and high RMS watts) and, measure it again.
But, there are still more things to consider. How that measures were made, because each frequential band as a direct impact in loudness (how do WE hear the volume, that is not a measurable thing contrary to volume).
The human ear is very sensitive to middle frequencies (because the human voice moves around 3K to 4K Hz) so, any bump in that range, maintaining the broadband volume will be heard as louder.
Still more things to take into account. That video is micking just 1 speaker of the 8 that a full stack has and, very close to the cab.
8 speakers move TONS of air, way more than 4 and way more than 2.
So, let that people measuring at 5 meters from the cabs and see what happens.
Also, put yourself in front of all rigs and, let me know which made you a new haircut and which one created an earthquake in your guts.