I will share my experience with my lastest bass heavy pair of IEMs in this post.
Thus far, my ears have never accepted PMP audio quality. My audio setup is at my desktop computer. I burn my CDs into 24/96 WAV files and run them through foobar2000 with 45 equalization bands available to a E-MU 0202 USB DAC with 24 bit capacity. The sound goes to the headphones through the DAC's headphones output which has an inner amplifier or through another external small amplifier. Right now, I am using the PA2V2.
I got the dual dynamic driver DENON AH-C300 in mid December. Those, have two 11.5mm drivers and are targeted to bass lovers or the EDM and equivalent crowd. When I first listened to them, my ears were neither amazed nor disappointed. Bass was abundant and of very good quality. Mids were very clean and clear with no mud in them. It was obvious that the effect of two separate drivers was showing in the mids. Still, lower mids were bassier than my audiophile ears wanted to tolerate. Middle mids were very good and so were the upper mids. Still, those upper mids had a tad of a cheap tonality to them. The highs were of very good quality in the upper region (10kHz and above), but seemed to lack a little in quantity. The lower highs were dry, sharp (harsh, piercing). With most recordings, I had to EQ down the 3-5kHz frequencies into the negative zone. Even with that, my ears were tortured with each snare and tom hit. My overall perception was that some parts of the sound spectrum were of very good quality and others were of mixed quality. But my ears told me not to worry, because those not so good quality parts had a good quality to them waiting to happen.
As I kept using my DENON AH-C300, I seemed to notice some changes for the better in those parts of the sound spectrum that had a mixed quality to them, mainly in the lower highs. Snares and toms would not sound as hard to bear as they did before. With more use, my ears asked me to move the 3-5kHz EQ sliders and they gradually ended up a couple of dBs in the positive zone. Now, after about 450 hours of use, I have to say "WOW, what a change, my ears knew it all from the beginning!!!! No part of the sound spectrum sounds close to being of low quality. My ears point at some parts where improvements can be made, but they smile and say "we will not complain with this sound quality, we are just letting you know what changes would make us happier."
What happened here? Part of the difference in perception is brain adaptation to the new sound. However, as this process took place, I stopped using the AH-C300 for several days and would go back to listening exclusively to the Rockit Sounds R-50 for long enough for my brain to get used to the R-50's sound signature. I did this several times. That kept my ears aware of the change that was taking place with the IEMs themselves. What happened with use, was that the big bass tightened up quite slowly, but surely. As it withdrew its presence from other frequencies property, those frequencies flourished as expected. Even the bass itself got better. Now, I have set the 200-50Hz EQ sliders as high as my ears always wanted them, and bass gets bigger and hits harder without affecting how the mids and highs sound. I couldn't do that before. Now, the lower mids don't have the note thickness that made them bassy enough to be regarded as an invaded zone. The middle mids are clearer and sharper. The upper mids (most vocals) lost that tad of shoutiness and cheap tonality that they had. The lower highs are a lot more fluid with much better decay and the harshness has been greatly reduced and they don't sound dry anymore. I can't say that the lower highs peak that is shown in FR graphs, got smaller. My guess is that when the lower highs stopped being dry by the bass withdrawal, my ears stopped perceiving those peaks as sharp (harsh, piercing) and now consider them acceptable or even good lower highs. The upper highs haven't changed much, but they were already good from the beginning. Decay (ssshh) is better and it seems that so is their presence or amount.
With this experience and a previous one with another pair of bass heavy IEMs, I am convinced of one thing: It takes 400+ hours for the bass to settle in its relationship with itself and with the other frequencies. This will have a significant impact on sound quality for all frequencies. I am suspicious that something similar happens to a lesser degree to the treble in regard to losing dryness and becoming more fluid with better decay. But, I can't be sure about that because the bass withdrawal is already having that effect on the highs. Whether only the bass change or both the bass and treble change, I find it impossible to fully trust any review/impressions of a bass heavy IEM model that has not been put to work for at least 400 hours. I know that it could be impractical for many to review IEMs in such a way. But, at that statement, my ears just smile and say "it is better to have a significant sound improvement than an accurate review, than to have an accurate review with no sound improvement." I am glad they are right. So, is my wallet.
