The JBL PRX635s are definitely a solid choice, and they do hold there value relatively well in the used market. From my experience, selling them as a pair might attract more serious buyers who are looking for ready-to-go setups, potentially allowing you to command a better price.
As for where to sell, platforms like Reverb and eBay are popular for pro audio gear. Revoerb specifically catifies to musicians and audio professionals, so your listing might receive more targeted visibility there.
Regarding upgrades or modifications, while the PRX635 is robust as is, ensuring they're firmware updated and incorporating excellent quality cables and power conditioners can sometimes provide slight enhancements in performance stability and sound clarity.
Hope this helps with your decision!