First of all, sorry if I bumped up an old thread, but I just wanted to put my opinion of this bass monster out there.
I just registered here to say that the JVC HA FX1X has got the deepest bass I have ever heard in my life, on top of the pretty much transparent mids and sparkly highs. Some would say the mids are recessed, because on many, if not all IEMs/headphones that I've tried, I often have to turn down the 1khz notch by around 3 to 5 decibels or so. With these however, I never had to turn it down, not even when its fresh out of the package.
And yes I can also confirm that the treble does open up in a short amount of time (25 to 50 hours I suppose, but at the time of posting, mine's already got like 2 to 3 weeks of heavy usage already).
The first time I put these on I just couldn't believe how deep the bass went. It's not a bloated kind of bass either; it was very comfortable to listen to. I've never heard anything produce bass in this manner before. It rumbles your eardrums but at the same time it doesn't have the bass (or midbass) hump that induces headaches in people. It's just very, very relaxing/pleasing to listen to. The bass on these is pretty much bottomless, and it goes well down to 20 Hz.
The JVC HA-FX1X also does come with its bad sides as well; the upper mids / treble.
Once these have opened up, the treble / upper mids start to become sibilant and tinny.
I opened up SineGen and found that it was peaking in several areas. There were 2 main peaks in the sound signature - one at 3350 Hz and another at 5200 Hz. There's also 2 more at around 7500 Hz (roughly, I can't remember where exactly this one was) and another at around 9000 hz. The latter two peaks are more or less harmless and contribute to the soundstaging and separation of instruments on the JVCs. The former two (3350 and 5200 Hz) however, were the culprits that caused some of you to have transient tinnitus after listening through these over an extended period of time.
These 2 peaks aren't just small peaks in the sound signature. They are extremely huge in amplitude (at LEAST 5 db spike; to my ears they sound like 8db spikes as compared to the surrounding frequencies, and also as compared to 1khz as reference). The peak isn't that large in bandwidth however; the 3350 hz peak for example stops completely at around 2900 hz and 3600 hz (by 'stop' I mean it goes back to 'normal' amplitude, again, with reference to 1khz and the surrounding frequencies).
Now, before anyone jumps the gun, I'd first like to say that this is also just a PERSONAL conclusion that I've reached about the flaws that the JVC HA FX1X has, which is also why I'd like people to also experiment and find out if my findings regarding the peaks in the sound signature were true or not (it might just be my ears that are picky).
Note that, fresh out of the box (i.e. pre-burn in), these problems were non-existent.
Before I end this post, I'd just like to point out that I'm NOT using my JVCs un-equalized (those peaks are just too unbearable for me), in case anyone asks. I mainly use the EQ to have those two main peaks (3350 and 5200hz) minimized, but not eliminated completely to ensure surrounding frequencies are not damped too much.
Anyhow, I would recommend these if you're a basshead and also have a good EQ to tune down those skinny but huge peaks in the upper mids/treble. The bass on these is just ridiculous for 20 dollars.
EDIT: Merry xmas and happy new years everyone!
I've always found that ridiculous bass is quite easy to find in the sub-$50 category range. The entire cheap IEM/headphone market is marketed towards heavy bass (and practically nothing else) and plenty of people seem content with that, hence why Skullcandy/beats/etc are such huge companies.