After having the HE-5LE, HE-500 and the HE-4 following my HE-400, I have no regrets at all coming back to the HE-400. Sure, there are some things the other models do offer. However, to a degree, the also have what can seem at times, a bit less bass. This though, depends on the type of music, etc. Plus, having the "5" series from HifiMan meant that I treated them more like a trophy as opposed to a headphone that I'd use daily. The final straw came when I looked over at my pair of HE-500s at the time and realized they had been sitting on my Woo Audio stand for about a month. To me, that just seemed crazy, so I put them up for sale. I used my Sennheiser HD-600s for about three months until getting the itch to try the HE-400 again. So far, so good. No regrets.
As to the extra cash, that's sort of a wash to me. My wife knows how much I love audio and making a splurge here and there won't cause issues. But, doing so every 6 weeks does turn up the heat a little. So, here I am with the HE-400 and thinking of myself as an audio enthusiast again. I get to tweak a bit, use the EQ a bit, focus towards one of the JergPad mods, etc. This is the fun part. The other models didn't really lend themselves to tweaking -- again, due to treating them like a trophy.
Ironically, I also read a piece recently on the HE-6 and the reviewer was noting a higher frequency spike that was a bit bothersome. That had me thinking a bit. Here, one is with a $1,300 pair of headphones and they would, or might have to perform a level of adjustments to get the sound into a tasteful zone -- depending on their particular tastes. I've read similar reviews on the Stax headphones, too. If the reviewer is objective, they'll state such items in their review and not just sugarcoat 100% of the review and make one think that the model is truly the best, and great at everything.