I really like the RN6, but it's definitely more of a midbassy and thick low mids specialist offset with a lot of low and mid treble attack/ energy. It can be "shouty" when it comes to female vocals and electric guitars, but it's very lush with male vocals and instrument fundamentals. The treble is definitely quite bright and energetic, metallic.
It's slightly more technical than Volur in the treble, and arguably in the midrange but Volur is one of the best bass in an IEM ever, and the overall tuning is more to my preferences. (Sure call me boring, but it's just a well-tuned monitor, which is plain, but offset with a really nice bass slope). Volur
needs extensive tip rolling to help linearize its treble response - the narrow and long nozzle paired with the Tia driver makes for a tricky upper treble coherency in some gear pairings.
If you have very healthy high-frequency hearing, the Volur's Tia driver can make things sound artificially shimmery & shiny in the 10kHz + air (but so does RN6, though EST's are more ethereal (forgiving) whereas the Volur's Tia is more dynamic (unforgiving)).
Here's how both sets graphed on my coupler with preferred tips (Volur has M15 & M20 squigs):
And here with matched tips (Volur M15):
For 90% of music, cutting the graph upper limit at around 12kHz paints an accurate picture, but I do believe that the huge Air boost does trickle down harmonically into lower registers of the treble, or at least morphs the lower and mid treble.
RN6 is a more versatile tuning out of the box for multiple genres, but Volur sounds more timbrally accurate and neutral. Volur has the advantage of more significant tonal shifts swapping through the Apex modules than the Fir's Atom modules.
RN6 has a great stock cable. Volur's is more generic. Ironically, I use the RN6 cable on my Volur and find the combination extremely dynamic and very resolving.
Hope that's helpful!