Great to see a reply so quickly, thanks!
We failed today in our effort to buy a final RP-160M that would have been used at the Center Channel position. Unfortunately, the dude on Marketplace didn't reply to our messages. Which never happened before.
Unless we see a different, affordable RP-160M listing this week... we'll go with Plan B, purchasing either RP-450C or RP-540C. Which opens up its own quagmire, as it's hard to discern whether any meaningful distinction exists between the two Center Channel models, notwithstanding some Tweeter/Horn upgrades on the 504C. However, I can't find confirmation whether this upgrade is a legit difference, or mostly branding. Otherwise the specs line up almost exactly the same.
If we do choose the 540C we'd likely go with Series I for two specific reasons: (1) price, and (2) the high-crossover freq on the Series I is the same as our RP-160Ms across all surround positions. I want speakers in all positions to have the fewest variations possible, to create one, big unified soundstage.
But for the life of me I can't find a solid declaration by users testing both 450C and 504C... confirming firsthand that (they find) the 504C sounds better than the 450C. In fact, a review by Erin shows some 2k freqs are higher db (in a bad way) than the rest of the upward curve, when testing. In other words, if Klipsch "fixed" the Tweeter/Horn with the 504C, that implies the 450C Tweeter/Horn needed improvement (which I don't know to be true either way.) And did the 504C "fix" the "problem"? I don't know.
In the end, we'll probably gruffly say, "We're not spending more than we need to, we're getting the 450C!" And then we'll totally get the 504C. The Matrix has us. Me, at least.