Thanks for the input, guys.
I do plan to move the center forward a bit so that the baffle is slightly in front of the shelf edge. I don't expect much of an effect, but it can't hurt. I'll probably do this and then do another calibration when the Monolith floorstanders are available (planning to buy them).
Those "before" plots were made by the MultEQ app (Monolith first, Polk CS400i second) and I use a pretty tight mic spacing for the calibrations, so the plots are pretty much on axis with the center.
With the app, if I apply correction to 800 Hz, the Monolith "after" plot shows a flattening of the response in the 200-600 Hz region, so yes, applying correction (XT32) is an option. I usually try to apply minimal correction to the LCR, by limiting the correction range to 350 Hz, but the Monolith center would need correction to about 800 Hz, if I want to flatten the region with dips.
Assuming the dips are from ground bounce, interesting that 2 different models in same location would produce different ground bounce effects and/or other cancellation effects. But, then again, they are different designs: a sealed 3-way vs. a ported 2.5-way.
I do not expect a flat "before" response in-room, although I have seen some better looking app "before" center plots from others. A guy with a KEF Reference center comes to mind.
BTW, the Monolith center sounds great - no complaints. I think is has a bit of an edge on the Polk for vocal clarity, but not a huge difference from my initial impressions.
Anyway, this is an informational topic for me, not an issue thing.