Monitor SPL vs. Full-Range SPL

The main problem with high SPL level is that it requires costly drivers that do not compress or exceed Xmax easily....this is expecially true in the bass and cost goes up almost exponentially as one desires high SPL at low distortion in the bass. However, this is rarely a criteria for consumer audio where levels are usually modest and do not have to playback music at realistic levels to artists/musicians or survive accidents such as a dropped microphone or as mistake in a patch panel.

See this Equal Loudness. It is clear that 120 db SPL at 20 Hz is equivalent in loudness to 85 db SPL at 500 to 4 Khz (the mid range). Clearly there is a huge difference in our hearing sensitivity across the audible frequecny range (remember that decibels are logarithmic)

Also note that volume level tends to influence the presentation, as you go up in volume our hearing sensitivity is flatter with respect to frequency.

In practice, this means that rock music that is supposed to be played loud will be mixed a little lean in the bass whilst ambient or acoustic stuff intended to be listened to at low volumes will be mixed with more bass. (this compensates for our hearing non-linear response with respect to volume level)

Cdc,

Indeed your hearing is most sensitive from 1 to 4 Khz...therefore sounds in this band will determine how loud you can stand it.

120 db SPL at 20 Hz is acceptable to your ears (your body feels most of it and ears are not that sensitive at this frequency)

120 db SPL at 1 Khz is definitely too loud to stand at a continuous level...even occaisional peaks at 120 db spl is starting to seriously push it for your ears (listening damage would ensue if you did this for an extended length of time)