That S-Center looks great on paper. 3-way, 91db sensitivity and 8ohm impedance. I'm guessing it sold for much more than the 520C's $80-$100 sale price. Good find for $40.Yeah the S30 is pretty good for an MTM design. Better if used vertically. Although we may not get the RC263 at that sale price , there are plenty of three-way design centers out there. I just bought a JBL S Center ( https://www.crutchfield.com/S-r5s5bu1CkGE/p_109SCNTR/JBL-S-Center.html ) used for $40. We will see how that works out.
The designs of the SVS, Emotiva, and Monolith are all three-way and likely a significant step up in terms of dialog. I don't know what the JBL S Center sold for new, unfortunately, so not sure it's a great item to compare. But guessing it was priced similarly to the 520C.
I think someone in the JBL 5 Series forum at AVSforum posted about owning one for awhile and still likes it.