I don't actually know if you're disagreeing or not?
Both... Saying a track has it's own loudness potential might be partially true, but from ME to ME the loudness potential would change depending on how they go about achieving it.
Perceived loudness differing at the same RMS pretty much sums up to different choices of EQ and/or saturation in the midrange and very little else,
It really depends on the track. Imo/e mid range is a big part of it, but low and high end can't be discounted.
It really depends where the energy is, and where it's needed and most importantly how it's achieved and distributed.
but a mix being pushed hard seldom takes large EQ changes gracefully
Agree.
so for things to "vary greatly" at the same RMS the mastering engineers must be diggin' in quite deep with EQ or saturation in various ways which would alter the mix in a quite profound way and that usually isn't what's preferred IMO. Other techniques than actually changing the frequency content might affect the perceived loudness a very small amount, but it won't make say a given -10 master seem like a -8.
I do not think rms numbers tell a whole lot about the actual perceived loudness of a track.
How someone goes about getting level through gain staging, eq distribution, compression and limiting has a big effect on the perceived loudness, even though the end number might be the same.
In other words, given the same exact track, one ME might be able to get that track to a average rms level of -10 dBFS before the sound starts to go south, where another ME using different techniques might be able to get it to -11 dBFS before it starts to go south, yet depending on how the energy is distributed, the track at -11 could sound louder than the track at -10.
As another example, say you have a mix and simply pull the threshold down on a bw limiter until the rms reads -10 dBFS. Take the same mix that someone like Ted Jenson mastered and is reading the same exact rms. They would sound totally different in perceived level, even though they read the same rms.
One of the tracks might sound like ass at -10, where the other might retain/gain punch and clarity at -10. The perceived loudness would be different.
I think picking a number and saying a track is "that" loud actually tells little about how loud the track actually is...