I own the FH7 and and did a quick demo of the SE846. From what I can tell, the bass of the SE846 is much fatter, rumbles more, and reaches lower than the FH7. The FH7 focuses more on sub-bass with less pronounced mid-bass and has a relatively fast decay making it sound clean yet satisfying (FH7 bass responds to EQ well). Mids of the SE846 are also fuller and decay more slowly than the lightning fast mids of the FH7. In the treble, the FH7 wins out in terms of detail, airiness, and extension although it is much more prone to sibilance. The FH7 offers a much leaner, more sparkly, and more detailed experience but can be fatiguing for long listens. The SE846 is more laid back with better sub-bass meatiness.
In terms of fit and isolation, the SE846 is better for those with small ears since the FH7 is shallow fit with a large shell and can fall out for some people. This difference in fit in addition to the venting of the FH7 also makes the SE846 a much better commuter IEM since isolation is leagues above the FH7. However, those with average or larger ears will likely have no problems with comfort on either. Both have solid build quality and interchangeable filters although the effect of the filters on the FH7 is more subtle than pronounced. Hope that helps!