Thanks, Dan and Dr. Bob! For some reason my mind completely failed to consider the possibility I was staring at a studio chassis.
As that video reveals (assuming either of you watched the whole thing), KOST was one of the L.A. stations back then that was meticulous about the polish of its on-air presentation and sound. To your knowledge, when this studio chassis arrangement was utilized, was the output of the STL ever sent through third-party boxes like Sequels before completing its journey into the transmitter-side 8100A and subsequent possible XT? Given their budget and careful crafting of their sound at the time, I would be stunned if KOST just had an Optimod running barefoot. But I don't know if there were any companion processors that would've been made, or could have been configured, for sitting
in the middle of an 8100A (as "exposed" through using the ST). All of the popular companion hardware I know about from that period -- Arianes, Prisms, Compellors, Dominators, 222As, etc. -- went inline ahead of the 8100A in every non-ST setup I ever heard about.
Edit: Can either of you tell what's directly above the ST in that video? The resolution is pretty poor, but that would be the right spot to put something like a StereoMaxx.
You can say that again. What the 8100/XT2 did was magically pleasing analog coloration-wise, and when other magicians like Lynn Duke built their own modification sorcery on top of it coupled with carefully chosen and tuned pre-processing (David's reference to the legendary fogged KRTH cabinet), the outcome had mesmerizing results on all that old music.
I really, really,
really wish Lynn would just appear out of thin air here on RD some day and post his complete, detailed recipe for what was going on in that cabinet.

If the oldies format is fated to go away as a commercially viable one, I think the knowledge of how that oldies-specific processing sound was created needs to be prevented from being completely lost to the sands of time. It would be wonderful if enthusiasts with a love for that era's music were able to take knowledge like that and re-assemble the same airchain, hotrodded K-Earth style, for high quality, high bitrate oldies streams, so future generations of listeners could enjoy it the way L.A.'s lucky audience did for so many years.