The RWA mod doesn't alter the sound characteristics of the player in any way at all. It may allow more of the player's own sound out because the signal path is cleaner, but having owned both I can confirm that they sound the same. Any perceived change to dynamics etc. would be a matter of increased volume output due to the lower output resistance.
As for impedance matching. There are 2 things to consider which will always occur, but may work for or against the ear/headphones:
- Any fluctuation in the earphone's impedance across it's frequency response will cause changes to the frequency response (sound signature) of the earphones if it is not significantly higher impedance to begin with. Generally these fluctuations come from multi-driver implementations which is why dynamic drivers like the IE800s fair better in this department and multi-BA earphones like the SE535 often do not fair so well. It's worth noting though that there are some dynamic drivers that do not have flat impedance.
- Every type of driver that I'm aware of will be affected by the damping factor of output impedance vs ear/headphone impedance. This is where the 1/8th rule is important to consider. If the difference between the output impedance of the device and the impedance of the earphone is significantly less than a factor of 8 then the player will not exert sufficient control over the driver to reach its peak performance. I.e. for the 22ohm output of the AK100 this means an earphone needs to be close to 176 ohm (22 x 8) or higher. The mechanical damping of the earphone (how stiff the driver mechanism is) can offset this, but it is best to maintain a reasonable degree of electrical damping as well. A lack of damping has very predictable sound results because it always creates looser movements of the driver and therefore more bass, and less detail and clarity in the bass, etc. Of course, this can benefit some overly analytical, bass-shy earphones.
Because of the impacts of personal taste which might prefer a modified frequency response and specific designs which might provide extra mechanical damping, it's impossible to know exactly what will / won't sound good, but these impedance factors are always at play - it's just the earphone-specific results that vary and our perceptions of these results for better or worse.