The general consensus says: no. The HE-400 apparently has more bass and more punch.
Hmmm the smoother treble and better upper mids sounds good....
But I would NOT like to lose the amazing bass and lower mids on the HE400.
I want to see a HE450 with the same bass and lower mids and slightly more upper mids with better treble.
But I guess this would be pricing themselves out of the market as it would possibly be better than the higher models.
It seems that the HE-400 only bests the HE-500 in the bass department because of its coloration. The bass is more prominent due to the upper mid recession (which also makes the treble more prominent). I could be wrong though...
Placed my order on a used-like new RE-400 on amazon for 89 dollars, no tax free shipping. Will write a very short comparison to the HE-400. My current Skullcandy Holua pisses me off and Apple buds don't provide isolation.
I know it's a weird comparison but they two are somewhat relatable. I think it'll be interesting how Hifiman will translate their house sound from entry full-size planar to entry IEM.
IEMs are just too different than full-sized to properly compare (apart from things like tonality) but I agree with Matt in that the RE-400 has a seriously good price to performance ratio. They require you get a proper seal, so make sure you've got the best suited tips for your ear...fortunately they now ship with a lot of choices. For the best bass response and isolation you'll probably want to use the supplied Comply tips or as hemi stated above some DIY tips made from earplugs. It's pretty simple to do, just cut 1/3-1/2 off the tip of the earplug, make a hole in the center and use some toothpicks to widen the hole enough to slide it over the nozzle. Still, they sound really great with silicone tips which is what I typically use.
True on tonality and the listener, I have to agree with you there on that being the most important thing to look for overall. Sorry to go off topic but I was just considering that with full sized headphones the sound is hitting the whole ear as it enters the ear canal, whereas with an IEM sound just goes straight in the ear canal, so overall sound presentation is different. If you had a full-sized and an IEM with the same FR, wouldn't they still sound a bit different from one another because of that?