Well, as you stated by mentioning it "subsonic", most people can't actually hear below 20hz but what it does do is add to the impressive vibrational effect you feel in your clothing or furniture or even body when thinks are cranked. Let me you that the EP500 does this very well.
Your second question kind of asks a couple of questions. First to give you an idea of what the 80db is all about, think of it this way. A lot of movie theaters are designed to present the movie so that the sound is hitting you at an 'average' of 85db. Just like in real life, sometimes things are much quieter and sometimes things get much louder. You might break lightly over 100db at the loudest parts of a movie.
Many receivers are calibrated so that in average conditions, seting the volume at zero will produce sounds at an average of 85 db. Some receivers may have zero set to 75 or 80, but you get the picture. This is what is referred to as a reference level.
Even if you were to leave it at zero, just like real life, many movies while have much quieter moments and much louder moments. Also, one CD or DVD can be very different from another. Between the differences in speakers, sitting rooms, and source material, 'zero' very rarely gives you the actual reference level.
In my case, my HT room is not overly big and kind of bright, I think. My receiver's reference level is 85db at zero. In my case, this works out wayyyy too loud for me. When set my volume to zero, test tones can be calibrate to exactly 85 but when I actually play a movie, most of the movie hits me at a very loud 90db or more. Peaks are hitting way too high over a hundred for my liking.
I usually find I'm set to -25 or so for regular TV and non-intense movies. For an action movie it might go anywheres from -20 up to -15. I have a couple of HD-DVDs that are inherently lower volumed as that seems to be the way the new HD audio tracks are being done. I think I hit an all time high of -10 when I showed off the Transformers HD-DVD to friends. I cranked the HD-version of my Nine Inch Nails concert to -10 as well. It just HAD to be loud!!!
Damn, long winded again. An editor would hate me.
To answer your second question, If a soundtrack was telling a sub to produce a sound at 10hz but the sub is only cable of going down to 50hz....
- Best case scenario is that the sub might be rated for 50hz but is actually capable of going somewhat lower, just it does so at continually weakening volume levels. In this case, there might still be some acceptable LFE but it might get masked altogether by other loud noises in the higher range.
-- Worst case scenario is that the sub will be totally unable to produce anything below 50hz so you will simply lose out. Most likely, it would try to do it's best but what you would end up with was a very bad, rattly noise that would be both distracting and very unpleasant. When this happens, the result probably will actually be much higher than 50hz because you are hearing the rattle and not the actual LFE that should be creating.
With great power comes Awesome irresponsibility.