As you've probably now figured out, amplifier and headphone impedances are two different things.
There are varying theories about the best way to bridge impedances between amplifiers and transducers, such as headphones, to obtain the best sound quality. A general rule of thumb though is that the amp should have an impedance no greater than about 1/6 or 1/8 the impedance of the headphones. It's not really engraved in stone though. And many people drive headphones with amps that actually exceed their impedance.
A lower impedance amp will, in theory, better damp the movement of the headphone's drivers, which helps to reduce distortion. (Why, I don't really know. But that's what the experts in acoustics tell us.) A higher impedance amp will be louder, but also potentially add more distortion. The threshold where that distortion starts to become noticeable to the human ear is supposedly somewhere around 1/6 or 1/8 of the transducer's impedance. Again, these are just broad generalities which may or may not apply to your particular gear.
If your headphones have an input impedance of 25 ohms and your amp's output is 1 ohm or less, then your gear should be well below that threshold. And you should be just fine from the standpoint of distortion. And if your amp supports headphones up to 600 ohms, then you should be ok from the standpoint of volume as well. Because a 600 ohm headphone is significantly harder to drive to higher volumes than a 25 ohm headphone.
The 32 to 600 ohm headphone impedance range given by Oppo is just a general guideline. If anything, it means that your amp probably has an overabundance of power for the headphones that you're currently using. And you should probably watch that you don't set the volume too high as a result. That power may come in handy in the future though, with higher impedance headphones.
For more info on the subject of bridging impedances and damping between different audio devices, see these two Wikipedia articles (which probably explain it much better than I can)...
https://en.wikipedia.org/wiki/Impedance_bridging
https://en.wikipedia.org/wiki/Damping_factor
In a simple amp + headphone impedance setup, the amp is the source and the headphone is the load. And the output impedance on the source (amp) should be lower than the input impedance on the load (headphones).
Many amps also have analog audio
inputs as well though, for connecting an audio player. And the audio inputs on the amp will have a different (and typically much higher) impedance than its headphone or speaker outputs. So the amp functions as
both a SOURCE for the headphones, and also as a LOAD for the analog player connected to it. As long as you are using the correct cables though to connect the two, the impedances on the amp's inputs should take care of themselves.
It's only the amp's
outputs where you need to pay a little closer attention to the impedances to ensure that you are not adding unnecessary distortion. And using an amp with an appropriate amount of power and volume for a headphone of a particular impedance.
If all the above specs are correct, then the damping factor (load impedance/source impedance) on your current headphones and amp is probably greater than 25. Which should be more than enough for decent sound quality.
