Pros: Bass, bass, bass. They do also have impressive clarity and detail throughout the spectrum
Cons: Hottttt treble, a bit big and clunky
I've just packed up the H300 loaner ready to send it on its merry way, thanks to H20fidelity for organising this. My impression largely accords with those above, a big V-shaped sound sig, with fantastic bass. The clarity and detail across the spectrum is great too, but the treble is too hot. A couple of folks have said they're like a DBA02 with more bass, that's bang on for me.
Build and aesthetics: Build quality on these is pretty standard, acceptable for the price but nothing spectacular. The housings are big and stick out of your ears like a couple of little corks. I would say they are strictly for audiophiles in this regard, but there are plenty of popular consumer IEMs that are as big so that's probably just me. I found the fit and comfort excellent. I prefer wearing phones cable down, it lets you put them in and out much more easily. They should add a shirt clip though, but having said that I didn't miss it too much. I didn't use any stock tips so can't comment on those. I also didn't try the mic-cable.
I briefly used a pair of silicons then switched to a pair of comply tips with the foam cover on the nozzle. Most of my comments are with the latter.
BASS: I'm an ER-4 type of guy, so the first thing hit my on these was the bass. It's big, deep, detailed and fantastic. The bass is beautifully detailed and controlled, and it does not bleed much at all outside of it's range. The bass is fast and tight and extends very deep. For me personally the bass is a bit too big to be called natural, it's great for something different from time to time but not every day for me. But I'm likely to be in a small minority on that, most will LOVE the bass on these.
MIDS: The mids are surprisingly good given the bass. They have that beautiful thick meaty tonality in the mids that make acoustic instruments sound truly beautiful. But they remain very clear and reasonably flat. You expect the mids to be recessed when you first listen to them because the bass is so prominent, but then when you focus in you're pleasantly surprised. The mids are not completely flat but they're pretty close. I think the mids are actually better than the DBA02, a bit more natural sounding.
TREBLE: Here's where these really fall down for me. With silicons these are hot, hot, hot. People who like the hyper-sparkle of the CK10/DBA02-type will have no problems. But even as someone who likes a bright, fast sound sig, these are too much for me. They are sibilant and harsh to my ears. BUT, once I put the complys on they calmed down a lot, to the point where I didn't bother to EQ them further. If I was using them longer term I might still play with the EQ but they were acceptable with the complys. In that respect I'd say they're superior to the DBA02. They have excellent detail and extension. But even when tamed, they are still a bit harsh and screamy in the treble. I'm not a graph guy but I suppose it's the peak others are talking about.
DETAIL/CLARITY/SEPARATION/SOUNDSTAGE: The H300 are excellent in all these categories. Detail and separation is truly excellent from the very bottom to the very top of the spectrum. Soundstage is excellent, not the best I've heard but no one will have a problem with them in this respect.
Conclusion: There is a lot to like about these, if you like big, deep and tight bass without sacrificing detail and clarity, you may well love them. The treble is hot but with minimal intervention that can be well-tamed.