If you seriously need isolation, IEMs seem like the clear choice when I think it through. I really only have experience with Shure and Etymotic models. I don't wear the olive-shaped eartips of Shures well, meanwhile Etymotic included several sizes of silicon and foam eartips which go a long way towards finding a long-term comfortable pair. Of course you can get custom molds, which should be awesome 
On IEM soundstage, I agree that there isn't space to create natural occurring soundstage. That said, also From personal experience (FPE? FPEx?), they can create very focused and pinpoint accurate imaging and reference flat FreqResponse, and there are a Lot less shape variables for a surround HTRF to account for with drivers in-ear, so soundstage could be processed into the sound well, and surround could work well with a broader range of individuals (if it takes in-ear driver positioning into account).
I just don't need the isolation of my ER•6i much anymore.
On IEM soundstage, I agree that there isn't space to create natural occurring soundstage. That said, also From personal experience (FPE? FPEx?), they can create very focused and pinpoint accurate imaging and reference flat FreqResponse, and there are a Lot less shape variables for a surround HTRF to account for with drivers in-ear, so soundstage could be processed into the sound well, and surround could work well with a broader range of individuals (if it takes in-ear driver positioning into account).
I just don't need the isolation of my ER•6i much anymore.
