Newb question-what is 'nominal' impedence? I noticed the T1 is 650 and the HD800 is 350 according to that-50ohms higher than their rated impedence...?

*ducks*

-Tari>ChuckNorris

Newb question-what is 'nominal' impedence? I noticed the T1 is 650 and the HD800 is 350 according to that-50ohms higher than their rated impedence...?

With nominal impedance usually denotes the impedance value around 1kHz where it generally is at it's lowest.
This also is close to the DC resistance in most cases.
For single driver headphones the nominal impedance is always equal or higher than the DC resistance.
In the higher frequency region the impedance rises because of the inductance of the voicecoil.
In the lower region the impedance rises (and at points where the headphone resonates) because it moves 'easier' and thus doesn't need to draw as much current. When the applied voltages is constant and less current flows this means the impedance rises.

The specs given by manufacturers are often 'ballpark' figures and for some headphones the impedance varies several percent amongst drivers.
common ballpark figures used are:
16, 24, 32, 50, 60, 120, 300 and 600 but in reality may be off by 10% or even more.
It isn't even uncommon to vary between L and R driver.
This isn't a problem as long as the efficiency is matched close enough.

Thanks a lot for the table! That is super helpful. I would have to say that I'm still confused why an amp like Project Ember sounds phenomenal with the HE-4 and why many users have stated its great synergy with the HE-6 as well. To me, it looks like the HE-4 and HE-500 would both be "under powered" while the HE-6 would be extremely under powered.

The Ember can put out almost 2W which is the power rating of the HE-4 and double that of the HE500 so both these headphones can reach about 120dB this is VERY loud.
The HE6 will reach 110dB which is also loud enough for most, to go twice as loud (that's +10dB) you will need 20W !
Will have a look about the HE-4 filter but when EQ does not remove the tiz, electronic filtering won't help. felt in front of the driver might but can't tell how much darker it will sound and might take too much off (as it also affects the highs next to the tiz).

Okay.  Where can I learn about all of the stuff mentioned in the chart?

Is there a good article that would explain it all in relation to headphones/audio gear rather than something more generic?

This old guy is always ready to learn something new...