How loud are you typically listening?

I typically use C scale, which is a flatter measurement. The A scale cuts both low and high frequencies to simulate the human ear's sensitivity at low to moderate levels.  The A reading is only flat from about 500 Hz to 6 KHz. As such, the A scale will appear to register lower than the C scale. Which one is more commonly used depends on the application.

The volume difference really is massive between the 2. A weighted 90 db is freaking LOUD. Measured C rated not so much. 
Is one considered a standard way to measure in a home stereo application? 

Mostly mid 60s, but as much as low 80s if I'm frisky.

@emrofsemanon There free options available at the Apple store such as dB Meter. Might or might not be super precise but close enough.

That’s an open ended question.

How loud do I play what? A Bach violin sonata, or God Save The Queen by the Sex Pistols, or somewhere in between, say Joni Mitchell’s Song for the Seagulls?

I would say that it ranges from 55 to 90, more or less.

And if your system doesn’t sound alive until you crank it up, something is wrong. The best systems - like a lover - whisper into your ear convincingly.