I guess the piece I don't understand is, "the sweet spot" ... what do you mean by this?
The RMS wattage rating does not tell you how the speaker will sound, other than how much power it can handle before it will distort above the tested threshold. Different frequencies also have different responses. I'm explaining things in a very simple way, for the sake of conveying understanding. But it's actually much more complex than this.
For example, the sensitivity rating is usually done with 1 watt power at 1 meter distance. But some shady manufacturers will do this at different power and different distances. You then have to really dig into the fine print. This really only applies to the cheap, low quality, manufacturers. No respectable manufacturer (ie: Hertz, AudioFrog, JL, etc ...) will do this though. Look at some of those really cheap Boss amps as an example ... the RMS and Peak ratings they assign are at such high % distortion, or done with an impractical test signal, that it doesn't translate to real word performance. This is why people dyno amplifiers, to see what heir real performance is.
Note the RMS wattage rating is also done with parameters. Like only one channel driven, or all channels driven .... also what signal was used to derive the distortion, and what % of distortion was it tested at?
For example, Sending a 1kHz test tone, to one channel of a 4 channel amp, and reporting the wattage which caused 10% distortion will result in a MUCH higher wattage rating than sending pink noise, on all channels, and reporting the wattage which caused .01% distortion.
So long as you stick with a quality brand (like Hertz, JL, AudoFrog, etc ...), you really don't need to be concerned with this level of detail though. You can just do the math on the amplifier's RMS wattage rating combined with the speakers sensitivity rating, to determine what loudness you can expect without distortion.
The wattage rating from these manufacturers is the limit you should consider. Although at 120w, on a 92 sensitivity speaker, is around 113dB. Add a second speaker, and it's now 116 dB. And of course, this is in a lab, without reflections (which cause doubling and well as cancellations). So you're real world dB levels can often be higher than this.
Now when you start to talk about subwoofers, things change up too, partly because the human ear percieves low frequencies at a lower volume than mid/high frequencies. It also takes more energy to produce low frequencies. This is why subwoofer speakers have a lower sensitivity rating than mids and tweeters, and also why sub amps need to produce a lot more wattage to balance the overall sound of the system.
Ok, so I think I may be overloading you with info?? LOL ... but I wanted to make the point that the underlying principals in play here are complex. In the end though, 50w provided to a 92 sensitivity speaker, will produce 112dB (assuming the ratings are respectable and valid by the manufacturer). It doesn't matter if the speaker's RMS rating is 50w, or if it 250w ... the loudness at 50w will be the same. The difference between the 50w rated one, and the 250w rated one, would be how much power it can handle, and in turn, how loud it can play without distortion.
