Most anything can drive 600 ohms reasonably well. If you like the sound signature or lack of sound signature of the amp, it's probably going to match well, unless maybe you're looking at something that intentionally boosts the treble even more.
It's just that most cheaper amps don't have the voltage swing and power to drive them loud. For around 110 dB SPL, you need about 43 mW (into 600 ohms), or equivalently around 5.14 V rms output, according to measurements at InnerFidelity. With more dynamic range compressed music, I doubt anybody will want to listen that loud with peaks at 110 dB, so in that case the requirements are not as steep.
JDSLabs C421 looks just a couple dB not powerful enough for that, considering the supply rails are +-7V and the output op amp is not rail-to-rail. (I missed seeing an actual spec, so I'm just going by those considerations.) For most people and most music that's plenty fine though.
On quick inspection I don't see much info on the iBasso P4 either, but claimed 600 mW into 32 ohms implies 4.38V rms into that load. If that figure is current limited, then into 600 ohms the max output will be higher, and that's probably enough. If not, or if they're being "optimistic" on their specs, then that's probably a little short as well, which again may still be well okay.