I moved from C20’s to S20’s a few years ago. This was due to a change of furniture in the sitting room demanding a change in finish.
The S20’s were ex demo and immediately better but it’s not a night and day difference in my opinion, they grow on you over time with extra refinement and insight.
The two speakers sound very similar with the S20 just doing the same things better. I didn’t need to change placement when installing them.
I was using an NDX/282/HiCapDR/250DR at the time. I now have NDS/555DR/252/SupercapDR/300DR.
The 300 completely transformed the speakers but I wouldn’t say the 250 was lacking in anyway, well not until the 300 was heard! You shouldn’t need to change anything in your system to get the S20’s sounding better than the C20’s.
It feels like something is wrong. Presumably you have checked that they are connected in phase?