I recently bought an ebay C4000, and the correlator and peak unlimiter didn't sound like my Phase Linear 4000 that I've used for 25 years. I've been going through the C4000, and C4000t schematics and looking at signals under the scope.
So far I've found the clipping in the summing amp and log amp that's causing the correlator to swish on noisy programs. Found a PCB layout error in the correlator right channel that limits its performance.
There's also redesign potential in the correlator, peak unlimiter, time delay, and maybe sonic hologram generator.