Those peak ratings are just as useless as amp max wattage ratings. Do you play music for 1 second at a time? If you do there are bigger problems going on here.
You are correct about what RMS stands for, but you go off script when applying that to a subwoofer. RMS is technically an amplifier rating. When talking about subs it’s taken in a different context. The RMS rating on a driver is meant to be a guideline for what to power it with in amplifier terms. The manufacturer 100% knows what the sub should take daily. The peak ratings are just marketing. They could say 10kw peak (and some do) on a sub rated to handle 1000w RMS. I’ll power mine at 1000w, you power it at 2500w, let’s see whose sub lasts longer and sounds better (overpowering will also cause the sub to reach its mechanical limits - very bad). Also, there isn’t any wizardry or black magic when designing/engineering subs, it’s all math and science. Completely calculable and known through material science, finite element analysis, computer modeling etc. Simply put, if a sub manufacturer knows the sub could take more power they would tell you. Power handling = money. It would be bad marketing to not “brag” about what your sub can do.
Running a single sub that is rated at 400w rms is clearly not an SPL burp vehicle. So we’re talking about a daily driver. If the sub is in a sealed enclosure then I wouldn’t be too worried about 500w. Beyond that you’re risking blown up subs. You don’t have to take my word for it. I contacted Audio Frog about their GB12. I wanted to know what they thought about me running them at 800w? The answer was, “that should be ok but set the subsonic filter at 30hz so you don’t damage the driver”. Below 30hz is my happy place. I turn subsonic filters off. So, I’m not the proud owner of a pair of Audio Frogs. I needed something that could take more power.
So, if you want to drive the sub beyond its ratings you should start filtering out the lowest frequencies… But, what’s the point of that? I want all the lowest ****! Why buy a sub if it wont get low? So go ahead feed that poor sub 1000w but if you want it to live for more than 30min I’d cut everything below 40hz. Then you can brag about how loud your system gets doing 44hz burps…