Depends what you`re looking for, scientifically speaking, human ears can hear 20-20k hz, however we can perceive lower frequencies by touch, If you want the visceral bass punch of a headphone, look for iems with a shallow insertion and a wide bore size such as the JVC FX700`s, most iems are made to isolate well so they insert quite deep, skipping out on our ability to feel bass, this issue exacerbated by the use of balanced armatures in most high end iems which move less air than dynamic drivers used in headphones. You mention sound staging, you must note that the er4s are known to have a small soundstage. This topic is very personal as everyone perceives sound stage differently, headphones undoubtedly have a larger soundstage, especially open or semi open designs, some vented iems will get close but will never reach the expansive sound stage of a larger circumaural headphones, the FX700`s are a good example again, having little isolation, large rear vents and recessed mids creating quite a broad sound. You also mention detail retrieval, now this is where in ears should have an advantage, being closer to the ear drum and in your case using a renowned ba with fast decay. What`s causing your instrument loss is probably the restricted extension of the single ba er4, a model I owned myself, which has pretty much no sub-bass and just shy of neutral mid bass. These iems are very detailed but also old and I find a lot of newer iems to have better sound quality and detail retrieval across the board. Audio Technica make some very detailed iems, namely the ck10 and ck100 (Grado like sound, hyper detailed and mid centric, more extended bass due to a good seal, use sony hybrid tips) which both sound fantastic with better detailing and vastly better extension than the er4`s combined with a relatively large soundstage of incredible depth due to their larger housings made of titanium which is good for resonance, etc. I also find the Sennheiser ie800`s very good in almost every regard except microphonics, they have good isolation, very good soundstage depth and width, balanced sound spectrum, the most extension I`ve heard from any iem and probably the best clarity too, they don`t slam details in your face lik AT models but they do actually carry more information, especially the slightly boosted sub-bass which is quite engaging and easily lost in noisy environments. The AT models can be found around your price range second hand, the FX700`s (aggressive sounding=good detailing) which I highly recommend are also around $200 or less depending on how good your deal is and the ie800`s will run you for a bit more but their price is decreasing rapidly, already half rrp new.
