Speaker efficiency vs. power requirements

Power ratio??? Never heard of it. What is it?
It appears the flaw in your reasoning is that speakers rated at 90dB can and often do play at much lower levels. By definition a person can hear any level of signal above the noise floor. Consequently think of starting at the whole system's noise floor - including the noise embedded in the software - not the efficiency rating of the speakers to discern your dynamic range. Also remember that the power ratings of amplifiers do not dictate output. The wattage ratings serve as one measure of an amp's prolonged sustainable capability without failure.

I DON'T KNOW WHO YOU WERE TALKING TO BUT BOY ARE THEY CONFUSED. Speaker sensitivity is simply the amount of db's produced as a reference (usually 1 watt at 1 meter). What your amp kicks out are electrical watts (you know the RMS variety) while what's (sorry for the pun) coming out of your speaker are acoustic watts. Even a very powerful 200 watt amp may in fact only have a watt or two coming from a speaker, after the inevitable heat loss from electrical to mechanical. The wattage of your amp must be taken in context with your speaker sensitivity, how far you listen from, what kind of music and how loud? You are also confusing dynamic range with the decibel scale. The decibel scale is a fixed number, 70 db is always 70 db. Dynamic range is just the ratio of a quietest sound to a loudest sound (also called signal to noise ratio). Don't forget that power ratings double with a corresponding 3 db change. Therefore a 200 watt amp has twice the wattage of a 100 watt amp and is only 3 db louder, which is about the limits that you can detect by ear. I have thrown out some basic stuff out there as I am mainly a biology major. I'm sure some physics major or enginneer can add plenty more.

Your math is Ok, but 90 dB is not the starting point for your dynamic range. Dynamic range is the difference between the softest and the loudest, and 90 dB is pretty loud to start with. 120 dB is EXTREMELY LOUD, equivalent to a jet taking off. 140 dB is the threshold of pain. Your million watt amplifier producing 160 dB would destroy your hearing. A more reasonable 60 dB range would be 40 to 100 dB, well within the capabilities of most hi end amplifiers and speakers.

Your conclusion about amplifier requirements is not valid. Your dynamic range numbers for both analog and digital are off. Analog tape can have approximately a 60dB range. I've seen several different numbers regarding digital, but the effective range seems to be 80-85dB. The listening room also comes into play. A typical residential listening room has a background sound level of 50dB (if you live in a rural area far from trains, air routes and highways you could get down to the mid 30s). So if you set you system gain such that the softest sound level is equal to the listening room's background noise level, then you would need to produce a maximum sound pressure of 135dB to fully reproduce the digital medium's dynamic range. Assuming it could go that loud, which is a very big assumption, a 90dB sensitive speaker would require a mere 32-33,000 watts of amplifier power. With its smaller dynamic range, analog sources would only require 128 watts of amplifier power.

In the real world the above number are not particularly relevant. The measured dynamic range of recorded music rarely exceeds 40dB. Pop/rock music is typically in the 10-15dB range. Any intelligent listner rarely would listen at room sound levels above 105dB. A 90dB sensitive speaker only requires 32 watts of amplifier power to produce 105dB levels.