Originally Posted by SEALBoy
I'm a little new to the audiophile scene... could you describe what I might hear differently with the back vent mod and without? I'm not sure what "sub-bass extension" is...
Sure, no problem. The HE-400's have a very linear frequency response (aside from the upper midrange) with stock pads. What this means is that the response (volume) is consistent with each frequency/pitch. When reading the graph, look at the green line. The volume is on the y-axis and the frequency is on the x-axis. "Sub bass extension" is a driver's ability to have a fairly flat response all the way into the sub bass region (really really low bass notes). If you look at the HE-400 graph with vents, you'll see that there is a bump at 50Hz and a steeper "roll-off" at lower frequencies. This means that 50Hz will be louder, more prominent, while the frequencies below 50 measure "quieter".
The perceived sound is a little subjective. The lack of sub bass extension in 'vent-mode' may not be easily audible. First, hardly any music (aside from electronic) has anything under 50Hz. Second, the vent also affects the audible distortion (the lines on the graph below the green line) which can induce a perceived amount of sub bass that isn't actually there.
From my own personal experience, the vent causes two major changes: more rumbly bass (50Hz) and more hollow. The vent provides for a more v-shaped response (bass and treble are prominent on the frequency response, as if it were a "V") which makes the midrange sound hollow and recessed. In the end, I opted to not use the vent mod.
Edited by Thujone - 2/7/14 at 12:08pm