Pros: Monster soundstage; good lows (though, when un-EQd, can be overwhelming); good highs; good build-quality; thick, high-quality cable; angled plug
Cons: Recessed mids; quite closed-sounding; bass will be overwhelming for many non-bassheads; quite veiled-sounding when un-EQd
Initially I was completely turned off by the sound signature of this 'phone. The warm, bassy signature bothered me. I did expect them to be colored, but not to this extent. However, eventually, with a bit of listening, I became accustomed to the sound signature—to an extent. After a while I just knew I'd have to EQ them in order to get fully lost in music played through these. I personally tend to stay clear of very colored headphones and speakers since a far from neutral sound signature can remind me too much that I'm listening to 'phones / speakers which can make it very hard for me to get lost in the music. But, thanks to EQ, all is now well and my brain is no longer reminding me that I'm wearing 'phones when listening to music through these. There are, however, times when the odd song, usually of the rock genre, will sound like utter crap through these and I'm reminded that I'm listening through 'phones. But, fortunately, nine out of ten songs I play through these sound great so it's not a huge problem. I've realized for a while now why some songs sound like crap through them: it's due to, I think primarily, recessed mids. Due to recessed mids, some music (mostly rock and metal, though definitely not all rock and metal) sounds like crap through them. But when the right songs are played through them—including songs of the rock genre—they can sound jaw-droppingly amazing and make me think "Holy Hell—these cost just $70?!" For some reason most classic rock I've played through them (Led Zeppelin, The Doors, The Velvet Underground, The Beatles, etc.) sounded great, and even beyond great, through them. Yet the vast majority of modern rock I've played through them (The Church, Wilco, etc.) has sounded subpar and, at times, barely listenable through them.
Apart from the recessed mids, I find the bass quite excessive but that "problem" (it's a problem for me, but certainly won't be for many people) was easily fixed with EQ. These 'phones do sound a bit closed and claustrophobic (they are, after all, a closed 'phone), but I expected that and it doesn't bother me much. The recessed mids bother me far more than the somewhat closed, claustrophobic, sound of these. Initially I thought the highs were recessed, but after much listening I've found that to not be so. The highs are actually, for the most part, great—though a tad shrill at times. I think that when these were unEQd the powerful bass is what made it harder for me to properly judge the highs (or, perhaps, the highs were indeed recessed due to the cans not yet being burned in yet?).
Aesthetically, these 'phones look great. They're quite a bit smaller than they look in photos and the box they come in is so small that, upon receiving my parcel, I was convinced I was sent the wrong 'phones; I thought for sure they must have sent me the HA-S400 or HA-S500. Additionally, the build-quality of these 'phones is great. Yes—they're made of plastic (as you should expect them to be considering how cheap they are). But the plastic is thick and I'm sure could sustain quite a bit of abuse. The cable also is of high quality (finally: a portable 'phone with a cable built to last! The cable is thick enough to assure me that I don't have to perform any mods to it to prevent it from fraying, something I hate having to do) and has an angled plug. Comfort-wise . . . they're fine. They're apparently a huge step up in comfort from the 400 and 500—two 'phones I've never heard—but they're not the most comfortable 'phones I've worn. I find the clamping force a bit too strong, but I was able to make it ever-so-slightly less strong by placing a few books between the pads and leaving them like that for a while.
Overall, these are a good bang-for-your buck, well-built 'phone that are now my go-to portable can. My go-to closed 'phone for general use is currently the CAL! (but may soon be the NAD HP50) since it's highly consistent and handles all genres nicely; in fact, I've yet to hear anything sound bad through it. But if the 680's mids weren't recessed they'd likely get more listening time from me than the CAL! I absolutely love the monster soundstage, smooth sound and almost everything else about the 680. But those mids . . . where are they hidin' at?!
If you're a basshead and listen primarily to rap or electronic you'll love these. But if you, like myself, have a more eclectic taste in music and aren't that much of a basshead, you'll probably from time to time be disappointed with the way they render your favorite songs. Unfortunately, some of my very favorite bands sound like crap through these, so if these were my only 'phone I wouldn't be too happy. But they compliment the CAL! well and when the right songs are played through them they sure shine!
I give these four out of five stars. I docked one star for their recessed mids (which makes them inconsistent and prevents them from being an all-rounder), their somewhat closed sound (I realize that they're closed but there are closed 'phones out there that don't sound closed; the CAL! is a good example of a closed 'phone that doesn't sound closed), their (rarely) shrill treble and their at times overwhelming bass (which, fortunately, is easy to tame with EQ).
I hope JVC will eventually release a portable carbon nanotube 'phone with less prominent bass and more pronounced mids.