Looking to compliment the ER4P/S? EX600, GR07 or RE262

Of the ones you've mentioned my friend, I've yet to hear the ER4P/S, but my first ever Etymotic in-ear (ER4S) should be here any day now. Nevertheless, if you're looking to compliment the ER4P/S's analytical sound, the RE252's are definitely worth a closer look IMO. Analytical, yet musical is the brief description I'd attribute to the RE252. I personally prefer it over the mid-centric RE262, and brighter (comparatively speaking of course) RE272. The RE252's mids are just what you'd expect from the high-end HiFiMan in-ears, simply exceptional. To no exaggeration, there isn't a single vocal track that the RE252 doesn't render beautifully and naturally. On that note, music just sounds so raw/pure and natural, it's hard for me to fault their signature/presentation. Not to mention, second to the SM3, the RE252 makes for extremely easy low volume listening. Coupled with the Zo, I can't even begin to tell you how enjoyable of an experience it is.

 

All praise aside, they certainly do have their weakness. For one, the sound is relatively analytical, with great emphasis on detail (supposedly much like the Etymotics), and hence, will not be to everyone's liking. Nevertheless, they're anything but 'dull' or 'boring' in that regard. Secondly, their build is rather questionable, since the rubber body is quite 'gummy' like. Their cable is rather thin, thought I haven't had them nearly long enough to be concerned just yet. Apart from the aforementioned, I really have no other personal gripes with the RE252's. As it stands, my only 2 exclusive universals are the EX1000 and the RE252's, and I'm as happy as a clam. 

For classical/stringed instrumental genre try Ortofons either e-Q5 or e-Q7

simply beautiful !

I have only the GR07 out of the IEM's you mentioned but like a similar mix of music as you and find the GR07 is an excellent all rounder that is especially good for Jazz, electric guitars, and vocals (especially female). I own the SM3, and have owned MTPC, and IE8 in the past and think the GR07 is better than all of them for what I just mentioned. It is kind of like the best of all 3 for me, losing only a bit of each of their best attributes but suffering from none of their weaknesses. For me it is the perfect blend of detail and musicality and offers the most transparency allowing me to hear things like urgency and emotion more clearly than anything else I own or have owned.