So on my amplifier, 0db is the loudest and -50db, where I usually listen is what?

-47db is definitely not twice as loud as -50db, of course.

I'm not confident in what I'm saying here, so please correct me if I'm wrong as I'd like to learn:

Human hearing isn't linear in terms of loudness. So a 3db increase in loudness sounds like "an increase", but the pressure is actually double. Hence, it makes sense to use db to describe loudness even in the context of perceived loudness to human-hearing.

This is similar to brightness. In photography, "stops" are used to measure brightness. One stop brighter is technically twice the light, but to the human eye, it just looks "somewhat brighter", as human brightness appreciation is logarithmic, just like "stops" and "db".

3dB is “twice as loud” in that it’s twice (or half) as much power.

Part of the reason why the bel is used is that human perception is closer to logarithmic than linear (weber-fechner law), so a logarithmic scale is a better approximation of “loudness” than a linear one.

Again, technically, it is. But ear isn't scientific device, neither is your amplifier. What I was describing is more than an agreement than some precise measurement. 3dB is more or less double the volume but different frequencies have different responses so I really wouldn't want to have some "perfect" way of measuring loudness as it would be so needlessly complicated that it would be useless.

As someone else alluded to, 3dB is a doubling in power, not perceived loudness. 10dB will be perceived as a doubling in loudness. This is the original unit (the Bel, rather than deciBel) which was I believe derived by testing on human subjects to measure this.

TBH I don't agree with a lot of the article - yes, dB on its own only indicates a ratio, but certainly in the field I work with this is known, and there are qualifiers (dBA, dbFS dbU) which tie the ratio to a known value so you're talking about an absolute, known quantity - even the dBa which is mentioned as if it comes out of nowhere is something which most audio engineers know about and use regularly because it's important to know the difference betweent he signal present and perception of it by the listener.

-47db is double the power. Perceived double loudness would be ~ -40db

I mean, it should be, unless your amplifier is taking its own liberties with the definition of dB.

You should have a legitimate question as to what dB’s the amp is referring to. What brand and model is it?

First, is it actually specified as dB’s? Most amps I’ve seen display volume on an arbitrary scale, no units specified.

Second, the amp has no way to know dB sound pressure, as that depends on the rest of signal chain.

So is the displayed figure referring to dBu, dB mV, something else, or something totally bunk?

Human hearing perception response is logarithmic. This is part of the reason we use the unit. Our senses naturally work in that domain.