I got these JVC HA-S400's based on the reviews here and elsewhere. I have a fair amount of experience with audiophile equipment at the low-middle end when it comes to speakers and amps, but I'm new to audiophile-grade headphones (I had some fairly high-end Sennheiser headphones years ago, but I never had very good equipment to drive them so I never really heard them at their full capacity. They fell apart a while ago --- the headband broke, finally).
Recently I upgraded my home DAC and the sound is astounding (Micromega MyDAC, I highly recommend) with my NAD amp and Polk LSi9 speakers, so I decided to see if I could reproduce this experience portably, but cheaply (because I had just spent money on my new DAC and associated cables/etc.) I got a Hifiman Express HM-101, which really does a remarkable job for a mere $39, as heard through various speaker systems. But I have to say I'm somewhat disappointed with these JVC HA-S400's. The sound is muddy (especially in the mid-bass), there's a lack of detail, and the highs just don't "pop". There's a boomy quality to the sound --- closed in. It lacks spaciousness.
I compared with my friend's Aiaiai Capital headphones (around $100), and the difference was stark ---much wider soundstage, more open sound, far more detailed, less muddy mid-bass, and clear upper register. Just notably superior in every respect, except for comfort (which is why I wouldn't buy these Capital headphones for myself).
The HA-S400's definitely improved with some burn-in --- but not up to the standard of a higher grade set like the Capital.
The sound is quite a lot better than "most" headphones I've tried, especially at the price point, but they're not audiophile quality by any means. I'm going to give the Monoprice 8323's a try, since they have the stamp of approval from a number of audiophile reviewers.