Also user settable EQ but 5 bands graphic only so more of a tone control than usable for correcting headphones/speakers.
RME ADI-2 DAC has parametric EQ, can be used for correction of headphones. K9pro only has 5 band graphic.
RME has clock slave, handy in a studio or when using more than one of them on the same source. K9pro can't.
RME has 6.3 + IEM output (both not balanced), the K9Pro has Pentaconn, XLR-4 out and 6.3 but no IEM (3.5) out all not balanced.
FiiO has analog in (RCA + Pentaconn) so can be used as a stand alone headphone amp or pre-amp with extra input. The ADI-2 DAC not (but ADI-2 pro can).
FiiO has BT input, otherwise the same digital input options.
RME has remote + informative display, FiiO has some LEDs and buttons, no remote.
RME has crossfeed + loudness + output level control + balance + mono + phase , the FiiO only a balance control as 'extra' and 3 gain settings (for amp).
FiiO has MQA, RME does not.
FiiO also has USB-C port (on the side, not rear) as well as USB-B, RME only has USB-B.
Output power ADI2 = 1.2W in 32ohm, FiiO 2.1W in 32ohm, RME and FiiO = around 300mW in 300ohm.
FiiO is somewhat cheaper than RME.
For reliability, and service the RME very likely has the edge. But as soon as the newest model comes out with a bit more power or lower distortion the FiiO is likely to get replaced within a few year anyway.