Yes, I have very same experience. But when you spend more time with customs, then you find out they are representing a true reference. Treble on the universal model is from some strange reason somehow elevated.
I believe mostly because of the fit and because use of silicone tips - they make any CIEM boosted in higher frequencies. When used with Comply foam tips, the difference is not as big in between custom and universal models.
Anyway after your ears / brain became accustomed to Layla customs, then there is no better option when seeking for true reference sound.
There is VERY little number of members who bought and listen regularly to Layla customs, Layla universals and other reference headphones / speakers. So when I read posts by some 'self-proclaimed' professionals who states Layla is not reference sounding, etc. .. it makes me really sad, because lot of regular readers and potential buyers will turn their heads away based on those misleading reviews. Jerry developed a real masterpiece and I'm truly delighted I can have such a sound in my ears anyway I go.
Interesting. To my ears the treble on the universal is perfect, but the bass on the custom, even at the lower setting, is overly boosted. I've confirmed with Astell & Kern that on the universal bass pot, 12pm is neutral. It must be that on the custom, 7am is considered "neutral". Anyone have word from JH Audio on this?
Unfortunately I'm not in a position to fairly compare universal vs custom because JH Audio screwed up the fit; after 3 refits, I realized something was still very fundamentally wrong so they re-shelled them based on the custom ear plugs I had them make for me last year, which fit perfectly. They're way better now --- I honestly wonder if they mistakenly used somebody else's ear molds the first time --- but not only do I still get the "somebody chewing in my ear" noise pretty easily after moving my jaw, they're now somewhat uncomfortable overall. I don't know why the earplugs they made are so spot on but the IEMs are not.
Anyway bottom line when I got my customs after the last, fourth refit, you put the overemphasized bass together with the fit discomfort and other issues, and less than 30 minutes after walking around with them, my ears were shot. Total fatigue.
I'm probably giving up on the customs, I'm tired of the refit saga (four so far) and for all I know have several more ahead of me. I don't know how typical that is.
I have to agree with your overall sentiment though. Our ears may have different opinions on custom vs universal neutrality, but for me, the universal Layla is incredible. I never thought I'd hear something so good from an IEM, and I've had Etymotic ER4P, Shure SE545, SE846 and custom Roxanne's. Layla blows them all out of the water. And paired with my shiny new AK380, they are simply exquisite.