Minireview and a mod I bought Hifiman-RE400 because I like their line of IEM's. I also have RE-ZERO and RE-272. So when I heard that RE-400 is supposed to be better than previous models I got interested. Is there any difference between RE-400 and older models? I would say Yes, in some ways huge differences. To me it sounds like the older line has a different housesound than the new line (RE-400). Older models are similar but with different variations. The new RE-400 sounds alike but it is different and it should because it has a newly developed driver and a new driverhouse. To me the biggest difference is in the way that bass is presented. The quality of bass is much better, more on the later. Main keyfeatures of this IEM I would say are these * Neodymium magnet * 8.5mm Titanium Diapharagm This is what I think is giving them their special sound signature. Neodymium magnet is the strongest type of permanent magnet made and because of this the driver should push serious amounts of air - and it does. So the quality of the bass is dependant on this feature I think. Titanium Diapharagm is somewhat known to produce great crisp treble with lots of detail. There are many that use titanium coated drivers in larger headphones but I don't know if it is something that is usually used in IEM's. I also find the size of the driver, 8.5mm, interesting. Bigger drivers usually sounds like they can provide more bass, but they usually loose speed so fast paced music can sound kind of dull. Then again if using a smaller driver size you probably would get the opposite - faster pace but thinner bass. So there should be a balance and I think Hifiman has chosen a very good value where the pace and bass is great. Also I like the use of aluminium housing as this should vibrate less and result in cleaner more controlled sound. So it seems like they have good features that should provide a great sound. So how is the sound..? Note. Not many state what dac/amp they are using when testing an IEM. Personally I think this is very important because IEM's can sound very different dependant on what you use. One important thing to look out for especially when using IEM's is the output impedance. Too high value can color the sound output so not all amps are suitable for driving IEM's. In this regard I am using O2/ODAC-combo (of course there are many more suitable combos). BASS To me this is where the new line shines. Bass was good with older models but not like this. It has a different kind of punch. It is very tight and controlled. This leads to the IEM being suitable for many more music genres. Previous models where excellent with some genres like pop, rock but many genres sounded a little dull and thin because of the bass for example reggae, dubstep, drum and bass, electronic music, hip hop. RE-400 actually handles all these genres really well imo. Bass is so tight and clean you can feel it almost. It is not boomy at all imo. As I stated earlier I think that the Neodymium magnet is responsable for this feat. On a last note I wan't to say that this is by no means warm bass. On some occasions the music can sound cold. This might be tuned this way because IEM's with warm bass usually are not that detailed. MIDRANGE Midrange is something that has gotten better with the later models and it is still good with the RE-400. As many has stated this is probably where this IEM shines the most. Vocals are clean and good. I can easily hear the lyrics even if the bass is pounding, not many IEM's that can handle this that good if looking in this price category that they are in (and sometimes even more expensive ones). I believe this could be one of the more neutral bass (as opposed to warm bass). To me the midrange sounds very forward, almost to forward sometimes because I sometimes sense that many things in the sound picture is way too back because of the forward mids. Imo it sounds like a n-shaped frequency curve (but I don't know because I haven't seen a frequency curve of this IEM). TREBLE Treble is also something Hifiman has handled great earlier and now as well. The titanium diapharagm provides crisp good quality treble, but imo it is too far back in the sound picture. So it's there but it is hard to hear all the details because of the mids being so forward. The bass also takes away some of the attention to treble. Earlier models seems to provide more treble but that is because how the sound signature of them are - either you have more bass and mids or more mids and treble. It is usually difficult to have all equally - somethings have to go (at least in this price range). SOUNDSTAGE Because of the treble not being as forward to me it sounds like some soundstage is lost in width. Of course this depends on which songs you are listening to and how they are mastered but in general that is what I hear. ANALYTICAL? Same here because of treble presence I don't find them very analytical. Sure there is detail but imo more treble presence would provide bigger soundstage and make them more analytical (like many BA-based IEM's) COMFORT To me these are the most comfortable of the RE-line yet. They are small so they go deeper inside your ear and you get a better fit. This should also provide a better bass response. CABLE This is probably my favorite cable if compared to RE-ZERO and RE-272. Almost no cable noise imo. BURN IN? I have used them about 50 hours or so. To me it seems like burn in changes the following: a slightly wider soundstage and tighter bass. So all in all I think it is a very excellent IEM. I like it a lot and think it is well worth current price of 99 USD. Imo this could easily be worth 149 USD as well. There are some slight flaws but I could not ask for more at this pricepoint. It can take on most competition in the pricerange. Sure there will be some that handles some parts like treble better but as a whole RE-400 should be the better IEM. ...and now to the fun part - mod (removed white foam / double black T4000-filters on top of each other) I accidentally dented one of the black T4000-filters when changing the tips that came with it (small bi-flange tips are my favorites) so I thought Oh well I can just replace it. But then I know of the foam mod on earlier RE-models where some say it changes the sound. Might as well do some testing - what this does to the sound. So I removed the T4000-filter on top and saw that there was a white circular foam inside the canal. I removed them both completely and did some listening. My predictions - it should provide more treble and bass should be very thin. This was correct. Then I thought the foam must be doing something to the treble that is why it's so laid back. So I tested what it would sound like when I have the white foam removed but put on only one of the black T4000-filters. Interestingly enough I heard slightly more bass, treble was perhaps slightly less than not using any filters at all but still much more presence than with white foam on. So of this I draw the conclusion that if I can encapsulate the housing more then I would get more bass, which is something that I like in RE-400 but I would also want more treble energy as this seems to make the soundstage wider and because of it being titanium coated I should be getting a more analytical sound like in BA-based IEM's. So I thought what happens if I put a second layer of T4000-filter on top of the one I already put. This should provide better isolation and thus elevate the bass as well. Well I tested this and.. wow. Bass was back to normal, really tight and punchy (not boomy) and I didn't loose much treble either. So in some ways by doing this I got the parts I was missing from RE-400 in the first place. It is like it has turned from something n-shaped to a more v-shaped frequency curve (although I cannot confirm this) - but still it is a little strange because v-shaped mostly means losing a little mids and I don't think I am losing any mids. They seem to be like before but now that there is more treble energy the mids does not seem that forward as before. Are there any downsides of this mod. Well, the bass is still not warm, but as I said before this leads to a very clear/detailed presentation of the sound. I also believe the aluminium housing slightly makes it this way. Now that there is more treble some might find it fatiguing - it all depends on what sound signature you prefer. I actually like it more now in that it is more analytical almost like a BA-based IEM. Mod as I see it should be totally reversible - I could just put the white foam back in as it was before and just close it with a T4000-filter (you receive 10 pieces of spare T4000-filters in the package). Maybe I can control the sound even more by putting on a third layer of T4000-filter, but I am actually perfectly happy with the way it is sounding now so I won't do that change. Unfortunately I don't have a second RE-400 so that I could compare them to each other. I'll probably end up buying another one. For 99 USD it is a steal to me. So to conclude this post - I think Hifiman's new driver is amazing and they should be able to put out great things of it in the future (for example RE-600 and maybe some others as well who knows) Thanks for reading. Remember that warranty is not valid if you mod them so what you do with the provided information is up to you.