PerformanceIn order to test these in-ears properly I used them both with my portable rig which consists of a Cowon D2 and a RSA Tomahawk, and some of my stationary gear. These in-ears are quite power demanding, but almost any reasonably good portable amplifier will be able to get them to sound good. I used them powered by just my RSA Tomahawk and it had plenty of power to drive them. Like with any dynamic based earphone I had the RE0s being fed pink noise for well over 100 hours, in order to be sure that it will perform its very best. Since the RE0s feature a radically different design than the RE1s I was able to get a good fit both from the medium sized single flange tip and the big double flange. The older Head-Direct RE1's shape meant that its plastic cable entry pressed against the lower part of my earlobe breaking the seal way too easily. The new RE0's design is just so much better in that respect. Just for fun I also tried them with my Travagan's Green amplifier, and it sounded good but with some noticeable white noise coming from the amplifier due to the RE0s relatively high sensitivity.
The first thing that popped into my mind when using these in-ears is balance. The earphones have the most faithful frequency response I have ever heard in an IEM. The midrange is forward and analytical without sounding harsh or fatiguing in the same way as the Yuin OK1s can be. The Head-Direct RE0s is a balanced beast that posses’ great balance between the bass, mids and highs. They have a more aggressive treble than the RE1s which is nice. Extension wise the RE0s are also way ahead of most in-ears almost on par with that of the Sleek-Audio Customs.
The midrange is incredibly detailed and gives you a nice sense of space. The midrange is forward without being dominant, this is one of the things that makes one perceive the RE0s as well balanced. Nothing is pushed back in the mix unless it was meant to be like that. This helps to bring out subtle details in the music because the earphones do not add anything to the sound. The sound becomes more believable and lifelike due to the neutral frequency response.
They are not sparkly in the same way as the Sleek-Audio SA6 or Customs with the treble plus configuration, but it has roughly the same amount of detail. Compared to the SA6s these in-ears do not have an elevated treble, so if you are looking for the most neutral sounding in-ears you should definitely give the RE0s a listen, there is nothing out there that delivers in this performance aspect quite like the RE0s.
So what is keeping me from going nuts about these incredibly detailed in-ears? For me the bass lacks a little more amplification, it is decent as it is but to my ears it is a bit too weak. Also the bass extension is not as good as that of for example the Sleek-Audio Customs. The bass definition is, like the midrange, impeccable. You cannot say anything really bad about them, they are just not super with regards to musicality. The sound does not excite you in the same way as the Sleek-Audio Customs and this is both a major pro and a con in some way, because it is so well balanced and lacks the natural loudness normal in-ears give you - some music will not sound as entertaining. What bothers me the most is the bass, a drop of bass more would elevate the experience making these in-ears sound heavenly. Everything else is just so brilliant, the sound is incredibly composed without noticeable dips, bumps, drops or anything of the sort that can distort the music. Add that to the natural midrange with near perfect tonal decay and you are in for a thrill if you like to hear every detail in the music.