What about the MACH 80? Would that one be redundant if you own the SE846?
The Mach 80 is more complicated.
When people talk about Flat/Accurate Studio Reference Monitors, they mean "How flat measuring speakers sound".
So you have an speaker that measures like this
And you want an IEM that sounds like that speaker... but there is one big question, at what volume? What volume is the speaker and what volume is the IEM. that is not simple to answer because even flat measuring speakers sound different at different volumes.
The Mach 80 is designed to sound flat/accurate, even at higher volumes. That means when you listen at an higher volume, the Mach 80 will still sound as flat.
The SE846 (and the TG334 and the Vision Ears VE3.2/VE7 and so on) sound flat at average listening volume and will start to have too strong bass at too high volume. But there are also speakers where this happens.
It is very complicated as the human perception of flat also changes with volume. What sounds flat to you at 70db will no longer sound flat at 40db (even if the frequency response did not change).
Flat is not as simple as it sounds (sadly), but that is the biggest difference. The Mach 80 will sound ""flatter"" at higher volumes but will lack bass at lower volumes.
The SE846 and others (including the Mach 60 by the way) will sound flatter at low/medium volumes, but will start to have too much bass at high volumes.