Originally Posted by bluemonkeyflyer
Completely sealing the cups (cup vents, jack, ear side baffle ports) = Less Bass, "thin" sound, and less efficiency (need more power to drive), IME.
Removing internal cup vent felt = Too Much Bass that bloats into the lower mids and knocks out treble.
Leaking caused by poor seal at cup-to-baffle rims, stretched pad flaps, poor seal at pads-to-your head = Loss of Bass
The density and weight of your cotton may be different from side to side. Get a $25 digital scale with calibration weight from Amazon.
More cotton, acoustic foam, fiberglass, felt, rock wool, polyfil, etc. = more treble, less bass.... and vice versa
There is manufacturing variance from 'phone to 'phone as well as from Right to Left drivers of the same set of phones. I often have to alter the damping materials in one side or the other to balance them. REW and a measurement kit, or a SPL meter, tells you if you're balanced, or not.
You could try un-sealing the ear side baffle ports and making a 4 mm wide modified bass port at the the center of just 1 of the 4 cup vents instead of completely sealing them.
Modified Bass Port = Controlled Bass aka less bass quantity, perhaps better bass quality
The pads you use make a Huge difference in FR.
Others do things differently with good results. There are many paths to good SQ but it will likely require a bit of trial and error tuning to balance them and get the SQ you like best. Don't give up...experiment!