This might be somewhat of a strange pairing, but I tried the Geek Out 450 in FRM mode paired with an Apex Glacier which I managed to get my hands on a for a while, and the pairing was really impressive.
The synergy was extremely good, and the sound out of my UE10Pros (a rather neutral CIEM) was very engaging. It retains the 'tubey' signature and dynamic edge of the Geek Out 450, while adding in the superb transients and expansive soundstage of the Glacier. I'm not sure how to describe it better, but the GO has a rather thick note presentation, which brings forth texture and instrument timbre nicely. The Glacier by itself is very sharp and fast sounding, with minimal decay, and what many term as excellent PRaT. With these two characteristics combined, the synergy is really really good.
This pairing works exceptionally well with EDM, rock, or any fast and heavy genres, but it might be a little too edgy for more placid genres such as classical or house music. Perhaps GO owners with the Glacier might want to give this pairing a shot.
The GO pairs better with the Glacier than either the O2 (doesn't really add anything) or the Portaphile 627 (a bit too slow and blunt) for my liking.
Anyway, that's just my two cents. YMMV. Cheers!