Which Rel subwoofer

I'm not familiar with either of those but I have a Rel B3. Great combo but I did replace the stock Rel cable. Better attack and decay. Which C1's do you have? Originals or MKII/Sig? When I upgraded from the original to the Sigs I had to cross over the Rel a little lower to keep the sound 'balanced'.

I combine Dynaudio 1.3SE with R205 (succeeded by the R218 I think).
In my room (rather small : 24 m2 +/- 250 ft2)it works fine.
If your room is larger and presuming the C1 has better bass than the 1.3SE, perhaps the R-328 is a better choice.

I think there very over priced for performance given.

All I can say is I've had several RELs and had reliability issues, stay away from REL!