I'm not sure where I mentioned it (possibly in another post), but I only had a short time with the HE400s. They sounded great, but do be honest, they were the best I've heard on headphones, so I never really had a good comparison. Honestly, my closest was some AKG 701s, or my Westone 4r IEMs - so, no comparison at all.
I'm definitely going to demo both the 400 and 500 using a Hifiman EF5 amp, as I already have the amp. Also, these will never be powered off a phone or even anything remotely portable. I have IEMs for that.
The mods sounds great, although I'm not sure how long I would wait before molesting a near perfect headphone. As far as modulor pads, are those the jergpads? And a headband mod sounds great.