What is the name of the tool?

Here is what it says in the McIntosh 501 owners manual.

• Illuminated Power Meter
The Illuminated Power Output Watt Meter on the MC501 is peak responding, and indicates the true power output of the amplifier. The Peak Watt Hold Mode allows the meter to temporarily stay at the highest power output and then slowly decay. The Front Panel Meter Illumination may be switched Off at any time.

I believe the meters are accurate. What I’ve learned from having them is I was amazed at how little power is used for normal easy listening. 1-2 watts does go a long way. Also when you crank it up I was amazed at how fast it can jump up and use several hundred watts.

I looked at the Parasound amps. Looks like nice stuff, I’m sure they sound nice. To me, they look a little plain. 

+1 On watching the meters. They are a bit of a throwback too. 

I’ve used a 1 ohm load resistor in series with the speaker. I measured the voltage drop across the resistor with an HP rms volt meter while measuring spl at one meter on axis with a B&K spl meter. Source is a sine wave generator. I^2*R=power. 

Back in the day, Technics made a SH-9020 Power Meter.  I not only sold them, but personally used one for quite awhile.  Pretty accurate if I remember correctly.

https://www.kenrockwell.com/audio/technics/sh-9020.htm