ASG-2 vs ER4S
Comparisons are done on a Leckerton UHA-6S.MKII.
Bass: On the lowest bass setting the ASG-2 still has more bass than the ER4S. The lowest setting sounds similar to GR07 bass quantity from what I remember, although slightly less. This lowest setting will be the setting for the rest of the comparison.
The G-2's bass isn't as clearly defined or as tight as the 4S. The decay is longer, and so bass notes meld into each other more than on the ER4S, although probably not more than the GR07. This elevated G-2 bass affects the mids slightly, accentuating lower mids compared to the 4S. There are, however, other things going on in the mids and upper-mids that set these two iems apart.
Mids: The lower mids are more accentuated on the G-2, resulting in a warmer vocal compared to the 4S. But things start to change as we move up the frequency range. While the G-2 curves downward, the 4S curves upward, leaving the 4S with overall much more forward mids. I picture an FR chart of the G-2 to have a slight dip from 2kHz to 4kHz, simliar to the GR07. So, vocals on the G-2 tend towards warmer than the ER4S not only because of the G-2's accentuated bass, but also because of the upward curve of the mids on the ER4S.
As for details, the G-2 doesn't make vocal nuances as apparent as the ER4S, but this could be due largely in part to quieter vocals on the G-2.
Treble: Treble is more difficult to assess. It does have a peak ~8kHz compared to the 4S's slight dip, but it is very similar in quantity to the ER4S overall, although I'd say the G-2 has a bit more. I believe the G-2's dip in the mids/upper-mids results in a more forward sounding treble comparatively. Since the mids/upper-mids of the ER4S is accentuated, treble of the same quantity won't sound as forward. What this means is that sibilance is slightly more accentuated on the G-2, as are loud cymbal crashes and snare drums.
But the quality and detail of the G-2's treble are very close to the 4S, although not as linear imo. Animal Collective's Brother Sport has a lot of complex sounds competing in the treble, and each sound was clearly delineated to my ears on the G-2, a feat the GR07 failed.
Overall Sound: The ASG-2 is not as linear as the ER4S. There are dips and peaks on the G-2 that do not exist on the 4S. Imaging, however, is just as good as the ER4. Layering and instrument separation are about equal, although they go about it in different ways. The slightly elevated treble and bass of the G-2 help to define individual instruments in their respective frequencies, while the ER4 uses driver speed and short decay times to define instrument separation. The ER4 sounds smoother as a result, and perhaps a bit more detailed, although not quite as layered, an effect of these iems' different approaches to imaging.
To give a better example, a cymbal and a low bass note on the G-2 will sound more pronounced than on the ER4, and the difference between them will be greater on the G-2. The same cymbal and low bass note are less pronounced on the ER4, and the smaller difference between the two sounds results in a bit less instrument separation, and the smoother sound I hear on the ER4.
I've reported my findings as objectively as possible, and will leave it up to the reader to make value judgements accordingly (although I'm sure I'll make plenty later in this thread).
Build: Build quality is flawless. This is the black rhodium plated ASG-2 with bass port and black cable. No microphonics. The cable is one of the best I've ever used, much better than the ER4S. The shells weigh less than I expected, only slightly heavier than the ER4.
... Our impressions of the treble are about opposite each other. That's also where I picked up the ASG-1.3 
