Sometimes, “isolation” is actually your hearing sensitivity lowering due to the physical pressure coming from ear tips. A custom IEM with an equal distribution of pressure will probably have an equal or higher level of isolation, but your hearing will be more acute, so it evens out. Filled CIEMs (whether acrylic or silicone) will generally isolate more than non-filled ones. Your Legend X also has vent holes for the DD, so that’s something worth keeping in mind.
Driver flex occurs in both universals and customs. It occurs when the pressure changes in a DD-equipped IEM as they achieve a full seal within the canal. This pressure change vibrates the dynamic driver, then it ‘pops’ back into place as pressure equalises.
So the reason I’m assuming I’m getting worse isolation is for an example like this... same level on my DAP for universals and customs, same environment... I can hear my dog barking downstairs at level 70 w CIEMs but with universals I couldn’t. I know it’s not the best test or example but it’s a real life situation that I experience every day so I was thinking that was a good indicator my isolation was worse w my CIEMs. I can also hear my oscillating fan in my room w the CIEMs.