Very interesting idea. Look forward to hearing what you discover. The opening on the J$ pad is 1 7/8" x 2 9/16". The angled part is 1 1/8" tall while the opposite side is 7/16" tall.
To clarify the post you quoted. The difference I noted were between my T50rp and D5000. When talking solely about the changes in the T50rp, I felt that the J$ pads increased soundstage, added just slightly more bass (primarily sub-bass), made the mids just slightly less forward, and I didn't notice much/if any change in the highs. Obviously they increased comfort too.
Thanks for the clarification and for taking the time to elaborate. I think the large size of the ear hole on the pad and your impressions fall in line with my generalizations although I am a bit surprised that the size of the ear cavity didn't result in a bit of a loss in detail or top end.
I put my prototyping on hold. I'm not convinced that a small, deep ear hole will improve on the SQ I can get from off-the-shelf ear pads like the 840s or Hifiman ones.
Before I go too much further, I wanted to see if someone could simply summarize what I can do to maximize controlled bass. If I dampen the cups too much with Dynamat and acoustic foam, will that impact the bass? Dynamat is pretty much a permanent install from what I understand, so I will skip it if it will kill the bass too much. I'd like a fairly balanced set of headphones, but skewed towards the bottom if it is going to lean any one direction. I have Shure pads that will help the bass from what I understand. I will also be covering up the vents with tape and then adding a small hole to the tape.
Also, I could only find stiff craft felt with a sticky back. Does anyone think that this will negatively impact the felt's impact on sound?
To maximize bass, definitely play around with the amount of cotton in the cups. (You might actually need more cotton to get the best balance.) Also, plug the small hole in the baffle but leave the slots on the cups unchanged from stock.
The adhesive on the stiff felt may impact the sound. The felt is pretty permeable to sound and air movement. But the adhesive may prove to be pretty impermeable.