As far as instructions/details explaining the mods I performed:
Grill mod (Credit - Jerg):
https://www.head-fi.org/threads/hifiman-regrilling-mod.619447/
Adhesive felt and Putty seal mod (Credit - ohhgourami): I won't link to this but just put "HE-6 mods" into Google
Mod groundwork and inspiration: credit to Bill-P
As far as the cables go, I'd strongly recommend Amplifier Surgery as they were made custom for me (desired length and connectors) using excellent materials (silver-plated OFC, 22AWG, teflon insulation, Neutrik connector, etc.) and for around $80. Check him/them? out on eBay.
In terms of mods, I would say perform the pad swap with the lambskin ZMF Ori first as it made the biggest difference for sure (both sound and comfort) and that pad seems to be the top choice right now. I was tempted to get the perforated lambskin since I'm a sucker for air and stage, but having heard the regular lambskins (in conjunction with the air-inducing grill mod), I feel like the perforated pads would definitely put the emphasis too far towards the treble end. After performing the grill mod, I took the stock grills and placed them over the modded ones while listening to see if I could hear a difference. The change is very subtle towards a more open/air with the mod, but if anything it was really cheap and easy to add a little improvement. I can't comment on the putty and felt since I'm sure I wouldn't be able to immediately appreciate a difference, but I'm sure it helps the overall sound here and there and was also cheap and easy to perform. Modding is fun and a bunch of different mods might not amount to much by themselves, but together can make a difference even if any measurements look minimal at best.
So basically, yes, I like so many others now consider the ZMF Ori (or similar) pads a must-have for the 4XXs and you might as well perform these other mods, too, since these cans deserve to be all that they can be. And again, because modding is fun.
Quick Listening Notes:
- The 4XXs (in their current, modded state w/ a couple hours of play) are not as even in overall response as the well-worn-in 400i I listened to a few months back. The 4XXs definitely have an exciting sound which is why I placed them similar to my modded Q701s I know very well
-Not as sibilant as my modded Q701s so fatigue isn't as bad with the 4XX although the clamp needs some lessening given the weight and pressure of these things. When you're used to AKGs though anything is heavy.
-Bass is present, tight, and impactful, but not as much body as the bass port modded Q701. This might change with some additional burn time though.
-Mids seem ever so slightly recessed on the 4XX, but the slightly less piercing treble keeps them similar to the AKGs.
-I feel like the 400i I listened to had better imaging as I noticed it right away as apposed to looking for it with the 4XX, but this could be placebo as some tracks exhibited excellent imaging on the 4XX.
-Soundstage: 400i (stock) < 6XX = 4XX (stock) < 4XX (modded) < Q701 < HD800.
I'm fortunate to be able to use my work's demo HD800s so I plan on doing a (relatively unfair) comparison next week between the 4XX, Q701, HD800 on my own equipment. I'll post my findings.