I consider the PFE and ER4 on similar levels. Both are very good but excel in different aspects.
For example, the ER4 has a great soundstage while the PFE is less spacious and with less specific location and separate. The PFE is more dynamic and effortless in sound while the ER4 is more tame and even. The ER4 has a balanced upper end frequency response but a little early roll off on the lower frequencies. The PFE has a very extended bottom end response but has a more ragged top end response. The PFE has very high detail and is light, crisp, and energetic. The ER4 has a fuller note and a slightly smoother sound. The PFE's more engaging while the ER4 is more balanced and non-intrusive.
While I see pluses and minuses for each earphone, it's tough to universally say one is definitively better then the other. It comes more down to personal preference. I personally consider them to be on similar universal levels, but I do tend to prefer my ER4S because it's better balanced and even. However, if my preferences were more instead for ultimate detail, sparkle, and strong bass, the PFE is obviously the better liked option. It all depends.
How are the Etymotics known for a
lack of sound stage? My ER4S has the best soundstage of any earphone I've used.
The space and placement is very good and the sound isn't stuck inside your head. I can't really comment about the ER6 or something though, but the ER4's sound stage is quite good and very much a step above the PFE.
As for bass, the ER4 ≠ PFE, even with EQ. You can work with the frequency response some, but the way the sound is presented is a bit different. As well, the ER4 has less output capability then the PFE and noticeably gains compression in lower frequencies when pushed and EQed up. The PFE can get quite a bit more output and can belt out the lower frequencies with much less restriction. I will say I prefer the note of the ER4 more then the PFE. The PFE is very light and crisp in note, short. And this makes for a light bass note. The ER4 has a fuller note but also has good punch and control. With EQing it can be more pronounced and somewhat extended. However, it seems to run out of excursion above medium levels, and you simply get no more bass coming as you turn up the volume. Mids and highs just get louder. I like the fullness of note of the ER4 better but the PFE is more energetic and effortless and louder even if lighter in note. Down to a point, the ER4 can match, but overall, the PFE simply is more capable both in extension and output.