Well, my Truthears arrived today! Tried them out, remember I'm an IEM noob, never even tried them before (even though I have lots of over ear headphone experience EQ'd to Harman Curve & additionally some studio monitors with a sub). I followed the instruction on how to assemble them & wear them (easy enough), and used the medium sized silicon tip with the narrow internal bore (which is the one Amir measured) and it happened to fit my ear fine - I think they did anyway because the BASS was absolutely the first thing I recognised when I switched on the music through these! Initial reactions were that this is the best or equal to best headphone bass I've experienced - it was there in the right amount and there was a lot of definition & detail to that bass.
The second realisation I had was the rest of the frequency response felt like it was very well balanced, and there was certainly enough treble, wouldn't have wanted anymore treble but at the same time didn't really want to take any away - the tonal balance on my busiest track that I often use as the first port of call to assess overall tonality (Supermassive Black Hole by the band Muse) showed that tonality was absolutely fine. My next thoughts, third thought, I was wondering if the IEM was bringing out all the different levels of detail that I was used to with my best over ear Harman EQ'd headphones - on this point I'm not totally conclusive, it's possible I can get more into the different layers of the music more on the busiest tracks with my best over ear headphones (say HD560s EQ'd to Harman). I did briefly listen to my HD560s on some of the same tracks and my first realisation was that the IEM's sounded a bit thicker - I think this because the bass is very completely represented in the IEM, and the IEM Harman Target actually does have more bass (to make up for lack of tactile bass vs over ear Harman Headphone Curve) - and my theory is that this enables you to hear more definition in the bass as it's simply at a higher SPL even if tonality does sound right like that.
Last thoughts - soundstage. My first listening of the day today was with the IEM's and I wasn't disappointed at all in the perceived soundstage & imaging, it felt natural and panning effects transitioned well around your head. When I was trying the HD560s afterwards though I did notice that the soundstage was better, indeed a bit wider, and I think it felt less flat in terms of it's shape (IEM was maybe like the soundstage layed out on an A4 sheet of paper somewhere in your head, whereas perhaps HD560s felt like it didn't feel like a flat sheet of paper and instead had a bit more backwards & forwards shape to it - quite hard to describe and it was a quick listening comparison.
One of the things that impressed me with the IEM (apart from the bass & overall tonality) was when I was listening to Max Richter Vivaldi Four Seasons Recomposed - the track Winter 1 - the violin strings & other strings sounded potentially the best & most balanced I've experienced in a headphone, and the deep bass of the cellos (I guess they were cellos) also felt very well represented, along with the atmospheric lowest electronic bass elements/effects he puts onto that track. Interestingly that's not a busy track as there's not millions of competing elements (loose reference to one of the points I was making in the previous paragraph.....maybe this IEM's strength is not in seperating out every different layer on the busiest tracks). I wonder if the violins sounded so good because this IEM has a very smooth treble frequency response (no sharp dips or peaks), and perhaps this smooth frequency response has more probability of translating to what is actually experienced at your eardrum - as perhaps over ear headphones have more potential for sharp peaks or dips due to interaction with your outer ear to converge further away from the smooth frequency response that was measured on the dummy head - dunno, some loose intuition!
A thought on EQ. I decided it didn't need anything doing to it, apart from potentially putting in a +5dB Q1 Peak Filter at 20Hz - I looked at Crinacles graphs for this IEM and this filter allows Harman Level bass at 20Hz whilst it virtually stops it's influence by the time you get to 65Hz, which is what's required to match the curve. It was 50:50 whether or not it was an improvement or not, eventhough it was obvious that it made a difference to the experience. On the plus side I think it evened up with the treble even better, and it also made the overall bass that bit more prominent, but on the negative I felt on some tracks that it clouded a bit of the musical detail of the bass, as in it potentially obscured some of the detail in that bass, which is likely to be more of a psychoacoustic phenomenon I think rather than a distortion related phenomenon as these IEM's have low distortion in the bass, and certainly at the relatively normal to low levels I listen at (I'd be below the red line in Amir's distortion graph). So I think my preference for with or without bass EQ is related more to what my ideal Target Curve would be rather than anything distortion related in the IEM. I can definitely take or leave EQ with this IEM, it's literally 50:50 depending on the track.
My final parting words in this post: I haven't measured these on my miniDSP EARS yet, so I don't have any channel balance information for you. But I will be measuring them over the next days or week, so I'll post again with the results....and it should also give us an idea of the bass level (for my pair) as well as how good the channel matching is. Overall I'm very impressed with this IEM, I think it's probably the best £50 you could spend on any audio gear!