What’s the relationship between gain (dB) and power (watts)?

SPL in your room, in your sitting position is also largely contributed to by the room itself.
There is a phenomenon called cabin gain or the "transfer function", I am going to quote from car audio information.

Have you ever wondered why car audio systems can have so much bass as compared to a home stereo system? The reason has to do with space. Your home has a lot of it and your car doesn’t. Because there is so little space in a car the bass notes (which are long pressure waves) build up inside the passenger area. To calculate the length of a sound wave you divide the speed of sound (in feet per second) by the frequency. At sea level, the speed of sound is approximately 1,127 ft/sec. For example, a 40 Hz note has a wavelength of approximately 28 ft (at sea level).

(speed of sound)/(frequency) = wavelength
OR
(1,127 ft/sec)/(40Hz) = 28.175 ft

Since the length of the average car interior (including trunk) is in the 12 ft range the 40 Hz note will be longer then the car’s interior.

This is why notes below 70-90 Hz (depending on the vehicle) will have a greater output than the rest of the frequencies. Once this magic frequency is reached, bass output will increase by about 12 dB/octave below that frequency. This phenomenon is called cabin gain or the "transfer function". So a smaller vehicle will have a greater cabin gain and should be able to have greater low bass than a larger vehicle. This is true for identical subwoofer systems with identical power.

The physics involved are true for listening rooms, and so SPL isn’t just about the amount of energy the drivers are feed or are producing, measured at the listening position. Many use control devices to attenuate and clean up low frequency nodes which can be attributed to cabin gain, or "transfer function". Boundary reinforcement is another interesting topic and is related.

I am sure it’s very well known by most of us, but often not hardly thought about. I may be wrong but it appears, MC takes this also into consideration?

It sounds goofy but at least it is clear I know the difference between volume level and SPL. Sound pressure level is what we measure with decibels. Literally the pressure difference between the compression and rarefactions of the sound wave.

Very simplified electrical info following:
One can think of voltage as electrical pressure (or potential force), resistance somewhat like a valve that restricts flow, and amperage as the physical amount of electron flow, for the sake of a simplified analogy.

Electrical work (measured in Watts) can be calculated many ways, a simple one is the amount of electrical pressure (Volts) multiplied by the amount of electron flow (Amperes).

As nitroxpro states, that it is an exotic alternating current, I would say for simplification that it also meets an exotic resistance (impedance curve).

Amplification can be somewhat described as if you had a small electrical signal controlling a valve on a larger electrical signal, as the smaller signal swings from positive to negative, in higher and lower amplitude from zero, it opens and closes the valve proportionally, positive and negative.

The amount higher than the source signal the amplifier potential has, could be considered gain.
Now it takes a certain amount of power to meter the valve (like in a tap), there is the impedance and the voltage of the AC coming in, if it's too little it won't open the amplifiers (tap so to speak) to realise it's maximum output.

The maximum output potential of the second amplified signal can be considered it's available working power.
Even if you hold the tap wide open, it can't possibly feed more power than is available, than the potential electrical pressure the amplifier can produce, through that valve.

I say there is exotic resistance because the components of the crossovers, the windings on the drivers are reactive to different frequencies, this reactive quality can be measured as impedance curve.

https://www.audioholics.com/loudspeaker-design/understanding-impedance-electrical-phase/page-2

Resistance is the term used to denote the opposition to the flow of current in a circuit or material. It depends on the resistivity and dimensions of the material. The resistance offered by a material to the flow of direct current and alternating current remains the same. It is denoted by the letter R.

Impedance

Impedance is the opposition to alternating current. It is a combination of resistance and reactance. It is represented by a complex number and has a real and an imaginary part. The real part represents resistance and the imaginary part represents the reactance. Reactance can be capacitive, reactive or a combination of both.

source: https://www.electricalclassroom.com/resistance-vs-impedance/