Okay so tried rockbox on Fuze.
I was probably biased the first time around since the lack of sub-bass and customization options really put me off, and the fact it does seem to have a sizeable midbass hump (which I've now removed) probably muffled a fair bit of detail in the original firmware. EDIT - Yep, feels like a huge difference when I dip 600Hz.
Honestly I think it sounds great, but it's just that it's the warmer alternative of the 5.5G. I think it still falls short in detail (not by a great deal at all) but on the other hand, nothing sounds harsh, even on my ER4Ps. So it's basically the same tradeoff that pretty much every headphone has. I can certainly see people liking one or the other better.
Another noticeable difference is the Fuze's soundstage seems pressed tightly against you whereas the 5.5Gs seems more distant. This gives the Fuze a more enveloping sound, producing a larger soundstage. On the other hand I think the fact that it feels relatively closer makes it more difficult to position instruments. Rather than it being relatively pinpoint, it sounds like it's coming from a reasonably large area. Again, there are certainly pros and cons to either. I tend to prefer better accuracy and more detail over the cons, but maybe I'm just used to it.