Finally, with some help from eBay, I was able to return the PL20 for a refund. Thanks anyway to everybody for your help!
Prior to returning the PL20, I also did a simple measurement of the frequency response of both mics (PL20 and RE20) with my limited possibilities. Since I don't have an anechoic chamber, I used a calibrated measurement mic (Beyerdynamic MM1) as a reference and measured all three microphones with RoomEQ Wizard, which is normally used for room measurements. All mics were positioned at the same spot at less that 1m distance from a monitor speaker in my control room. The attached plot shows the results (green: MM1, blue: RE20, red: PL20).
Since the MM1 is supposed to be pretty flat, it only shows the influence of the speaker and the room at that position. Any deviation from this should be attributed to the microphone.
The response of the RE20 resembles the official curve published by EV: a peak around 8 kHz and a dip around 4.5 kHz. Below that frequency the curve is rather close to the measurement mic meaning a pretty flat response.
Now the PL20 shows a strong peak (~+10dB) around 3kHz, which is neither present in the measurement mic, nor the RE20. This is also in line with what we were hearing in the samples I posted initially. Whether one can conclude from this that the PL20 is defective, I don't know. But it is certainly strongly out of spec. Just thought it would be interesting to post.