Well - I gave up on the weight issue and returned to an HTC device. I could get hold of a cheap used and worn out Vodafone VDA V (branded HTC S710 aka "Vox"). This now works as expected - despite I have to say that the noise level is still higher than on my old devices. Since the maximum volume is however high enough to compensate for a little attenuation in the music-adapter to get the noise to a low-enough level for the physical environment, it does not matter much.
If anyone gets hold of information how the BenQ E72 headset volume can be increased beyond the poor original state - let me know, please - I am really willing to give it another chance then. You can contact me via this blog or in the linked forums on the left side - my user name is tobbbie there.