Well, it's a bit early to be definitive as I am burning in the DD but there are clear differences in tuning starting with bass head bass power on the FD5 and massive soundstage I am not sure I heard such a big stage since my LCD-i4 and also to a lesser extent the IMR Rah and my audition of IER-Z1R which I would need to A/B but instinctively I would say aside from LCD-i4 which arguably is not even an IEM I never heard such a stage.
So for starter the stage alone and bass power just sets the FD5 apart. It really sounds like a full size headphone from stage and bass... it's almost puzzling and awe inspiring. That with the default bigger nozzle installed and the balanced tips and I haven't rolled cable either. FiiO certainly engineered something quite unique sounding or at least in a very select category in both bass and stage respect. Now on the flipside, the Volt's treble sets it apart in its own way with its transient speed and amazing extension. Volt is much more resolving to my ears although the FD5 is much better than I expected in that respect and it's not the final word either as the driver is just at 12h burn in and I expect things will improve.
In terms of tuning, Volt is smoother in every respect. Bass is not as punchy or powerful, although it's quite excellent the FD5 has massive bass presence and power which shadows the Volt. The mids is fuller bodied on the Volt, notes have more weight and textures are richer and upper mids are on the safe side. The FD5 mids have less body and weight both by way of tuning but also probably its semi open nature. The upper mids have considerably more bite and presence with only the slighest hints of sibilance on vocals as well on most sensitive tracks. They are not harsh but I could see some people having issues with the FD5 upper mids although I have to do some source/tips/nozzle/cable rolling before making that a final call. Treble wise, the FD5 has more lower treble energy but I like what FiiO did there, no treble hotness IMHO but an engaging energetic lower treble. Now upper treble, it's quite extended on the FD5 and presence is just a tad emphasized but not too much as to not make detail more prominent than the overall musical picture if you will. Volt is in another league IMHO but that's only fair given the price difference and the tech advantage of EST done right.
If I had to simplify a bottom line which I don't like doing but can be helpful, FD5 is more energetic with more present, punchier bass, brighter mids with more bit and more energetic lower treble making the FD5 brighter overall but not bright in absolute terms. Volt is the more relaxed of the two with its own unique appeal of smooth, relaxed and resolving with great body and weight. Now imaging is an interesting comparison where FD5 has a much bigger soundstage especially its outstanding height and better depth so the image occupies more space with more separation in absolutes but I feel Volt is just more precise which makes its imaging more accurate. Again, take all this with a grain of salt as I need more time and rolling between sources, nozzle, tips, cable