I don't see how this could be possible, as a matter of physics.
I don't know about this. I don't like foam tips because typically I lose treble, especially the complies. I prefer silicone, but the insertion has to be right. You won't run into fit problems with customs, although you have a small chance that customs may need adjustment after, but that doesn't happen often as long as your molds are made the way asked, and they don't do a crappy job creating your customs(which I'm sure they would try avoiding as much as possible).
I can't explain it scientifically, but to me, the customs create a more controlled sound, and seems like imaging is more apparent(this one baffles me). I haven't tried the top ones yet, but based on ones I've tried, it does sound quite good compared to universals IMO.
You can only be skeptical until you try customs. I was skeptical also.
I've experienced this with my new customs. The mid bass(where mids and lows collide) was covering up the clarity on the vocals. The vocals were recorded sounding muffled, you know when the mouth is close to the mic? So it was a good test track to see if my HE-6 can pick up more neuances, which it did. This thing will pick up things that I didn't know existed. After I noticed my new customs had some of these details buried in the bass. So it sounded more muffled. I tried to EQ to bring the bass down, and the vocals became more resolved. So yeah, bass can bury the details. So, I don't believe in JHA philosophy of moor bass. I guess that's why he spikes the treble while he's at it.
This could be also do to my HE-6(planar magnetic) having very noticible separation of sounds. Bass sounds separate from mids and highs. This separation is hard to achieve with iems I believe. Not sure, if any of you heard good separation with iems, I haven't yet.
I find this statement odd beyond measure - Once my customs are in, I never touch them until I take them out - Unlike any and all universals I own - moving around can dislodge a Universal, not so a custom - at least all the customs I have owned - and to date I have had 5 pair. I can smile, talk, laugh, drink, turn my head, and chew pasta without breaking a seal. I brought a bowl of spaghetti to the audiologist to make sure of that last one .
But seriously, I don't find that a custom has more bass, less bass, whatever treble.... - What a custom does for me is provide me an uncluttered sound - there is nothing blocking the sound wave exiting the tube before it hits my eardrum - no squished foam or silicon (Sometimes I wonder how any sound gets through the thin passageway on the included foams from my universal Westone 3) - there is no requirement of cocking my head a certain way to get the IEM firing properly - customs (at least for me) disappear in my head - I forget they're there.