I was skeptical that much could be improved in terms of sound and comfort when spending 50% more over the MSRP by going the ZMF pads route. Considering how decent the price/performance measures for the HE4XX's sound at stock, there isn't much more to be done without putting oneself into a higher tier headphone. Rarely worth it, imho. I was pleasantly swayed from that thought after I added the ZMF Auteur leather pads to them. I mistakenly ordered them over the Ori pads that are popularly applied by Hifiman owners as aftermarket pads, but after a short email exchange with Zack from ZMF, I kept the order as I was curious if they would display any sonic changes similar to when pad swapping the Ikon's stock pads with the Auteur ones.
While the Focus Pads A are already better in every way to the stock pads on the HE400i (only other Hifiman's I've heard), the Auteur pads open up the soundstage noticeably, without any detriment to the frequency range as far as I could hear. While the bass amount was a tiny bit more abundant, the extension seems to have deepened, and I think I'm just touching a bit more sub-bass listening to CMYK by James Blake. Max Richter's Vivaldi Recomposed revealed an improved instrument positioning when compared to the stock pads. The ear openings on the Auteur pads are a lot wider and oval shaped, when compared to the Focus pads so curious if these have anything to do with it. I don't have the gear to provide an objective measurement so all these impressions are obviously very subjective. While I don't think that spending so much on upgraded pads over the already well-performing stock pads are worth it, they are certainly an improvement to my ears if money was no object. Still, maybe going for a better cable (the stock HE4XX cable is usable but begs to the upgraded) is money better spent.
The improvement that was much more noticeable tho was in the comfort category. Wow these are luxurious. These pads, in combination with the ZMF Pilot pad, makes the HE4XX cradle my head like a mother's bosom. When my wife put them on (most of her listening is done with modded Sennheiser HD580's) she immediately noticed the comfort change ("Not better, just different"). If you are looking for headphones that fade away, this configuration does not facilitate that. However if you are looking to feel a physical connection with the music apparatus, this does the trick. As much as I love listening to music, the experience of equipping oneself with such a great piece of gear sometimes adds to the overall listening experience. These headphones, when mated with the full suite of ZMF pads, just melt onto my head like a liquid gold crown, and there is no way I'm going back to stock after the upgrade.
Worth spending the money on? Probably not. But I don't regret the upgrade one bit, now that I've had some extended listening time. The HE4XX's are a great value, and while I would always go for the HD6XX for a first 'audiophile' level headphone due to their durability, comfort, tool-less field serviceability, and scaling improvement as one upgrades other gear (amps, dacs, sources, etc.), the HE4XX's deserves to be shortlisted on the virtue of price/performance alone, before making any decision to jump deeper into the hobby.