I have only read about what a great Center the 504C is and I also have seen other reviews saying it matches really well with the 590's. I will have to look at other Centers though. Thank you!
The 504c is not that good and for the price you can do much better. I tested it as well as the RC64 iii among MANY I have been testing for my 590s (Stonehenge Cinema ie..6x JBL 590s).
I am on my final stages of testing with the final 3 (unless I decide to give a Tekton Pendragon a try) are the Monolith THX 365c, the Infinity RC263 and the Revel C208 (way more expensive than the others).
I tested the following
JBL 530 as center (vertical)
JBL 520c
Klipsch RC64, 504c and RC64 III
RSC200
Paradigm 600c
JBL 590 vertical as center (not an option for me right now bust maybe in the future)
Emotiva C2+
Elac UC52
Monolith THX 365c
Infinity RC263
Revel C208 (arriving today!)