Yes, sealed inner ear. and Yes for the flat 20Hz to 20KHz.
Just for the record: It's my understanding that the balanced armature transducers used in IEMs are not flat out to 20k -- no matter what some manufacturer specs may claim. They have response up there, but it's not flat above around 16kHz.
A vote here for the Sensaphonics 2MAX for best isolation (and thus lowest noise floor), most persistent seal and full-range, reference-type neutral response. Soft silicone earpieces are also the most comfortable for long sessions. For live FOH, the 3D Active Ambient is really cool if you can afford it -- lets you switch between console and ambient sound, which can really help save your ears.
(Full disclosure: Sensaphonics is a client of mine. But that's because I love their design.)
