I have used all the earphones I've mentioned.
I'll clarify on sound stage in my definition.
First off, stage size is not a major concern for me. Many times this falls into one of a few categories. A small sound stage is typically close/personal. You might feel like you're up on stage with the band rather then out in the crowd. A good example is Westone's UM3X, very much sitting right in front of the singer's face. A big sound stage is spacious but also far away. You feel more like you're in a concert hall way bay. A good example is Sennheiser's IE8, very well known for this.
A third place that sound stage falls into is distancing. This is more of an ability for an earphone to sound either closed in or limitless in size. The Triple.Fi is a good example of a closed in sound. It has good locational cues and layering, but all sounds seem confined into a room of a set size and no sound exists outside of this virtual room. Ones that I feel do better are ones that sound limitless where noises can exist close or very far away. I'll note the sound stage can sound small or big and still have this. The limitlessness is not constrained by proximity. The Westone UM3X is a good example of a very close stage presence where you sound like you're right up on stage, but at the same time sounds are not limited in distance where you can hear a noise that sounds 20 feet back or the crowd chearing 100 feet back. It doesn't sound closed in.
Proximity is a matter of personal preference. I kind of consider a closed in sound a flaw, a limitation of dynamic breadth.
The sound stage is also broken into location and distance. A good earphone should put every sound in a very specific spot in mental space. The UM3X is awesome at doing this. The SE530 is very good about this. Distance, depending on level of detail and linearity of dynamic range can be sometimes fuzzy or misplaced. For example, My Ok1 buds are very linear but slightly fuzzy on exact distance. The SE530 is messy and inconsistent on distance. The UM3X is very accurate on distance to where you can basically break out a virtual tape measure and say the drumer is 23ft. back, not 22ft. or 24ft. Layering comes from this ability to distance as well as the ability to clearly define complex, noisy information. Some of it has to do with the speed and accuracy of the earphone.
An earphone with a good sound stage can be of any proximity (size) desired but should be open (not closed in) and should be able to place sounds in an exact location in virtual space.
The ER4S is one of the better earphones I've used that does this. The UM3X is one of the best. The RE0 is incapable of creating locational cues. You get a vague sense of left, right, and center but you get little sense of specific location and distance. Layering is mediocre too. The sound stage size is medium, and it doesn't sound closed in. The RE0's stage presence simply isn't great, plain and simple.
I haven't really mentioned cheaper earphones here because a lot of them simply aren't that good with producing an accurate or even coherent sound stage. Some may sound big and spacious, but they also completely lack specific locational cues.
True Blue, I can't say I've liked any earphone for sleeping. I've found nothing that really lets you lay on it comfortably and without worry of damage to the earphone. I guess I'd sort of advise against it if possible.
Since you've used the C551, you'll have a decent idea of what the C751 sounds like, although I don't know how they vary in sound. The frequency response is nearly identical, but I have no clue if the presentation is different between the two.
The Phonak PFE with gray filters has nearly the same frequency response of the Etymotic ER4. The PFE is an earphone that is very comfortable and if I were to pick one that I might sleep with, it'd be one of them. However, the sound stage is only ok. The ER4 does better, but it isn't one you'd sleep with. The Westone UM2 would be a good choice too, sharing the same driver as the ER4P but with more bass, and the physical design is comfortable and sits flush enough where you could sleep on your side with them. Bang for the buck, the UM2 might be the best option for what you seem to be looking for. However, pricing new seems to suck a bit right now. For reference, I think I paid somewhere around $250, maybe a hair more, for a used UM3X, but new is well above $300, like $370+. The UM2 new is $300, but I would expect a used pair to hopefully be under $200. One can only hope though.
