Hello,
might it be useful to add some ceramic capacitors (4.7 nF on C21, C22, ZD1 and ZD2) like shown in the Pic?
Not really.
It cannot harm to install them, however, no one will hear their presence (or absence).
Many thanks for your professional opinion. Nevertheless - i’ll mount them as a “preventive measure”. 
OT
Btw … My Kit includes two double RCA Sockets - one of them seems to be the wrong Type. ( Input - Left Channel [L] is red Coded ) Isn’t it?
/OT
Thanks for reporting this, we’ll look into it.
Meanwhile, you can request a spare connector by opening a support ticket.
Sorry for the inconvenience.
http://support.velleman.eu
done! Many thanks for your help.