Quite a different setup then compared to the WiiM Amp (this thread is in a WiiM Amp sub-forum). The WiiM Amp - contrary to the Pro Plus - has it's own bass management with high pass filtering for the mains.
My recommendation regarding RC with the Pro Plus remains the same. Do it with your complete system pre-adjusted and running the sub.
Edit:
Did your friend also hand you over the measuring hardware for the DD+ 12?