Two Preamps Into 3 Channel Amp

You can use the MCA 325 to power the center channel. But how you connect the front L and R channels? Do you have two pair of front L and R speakers, one pair for HT and the other pair for stereo playback?
It is OK if the XLR and RCA is not connected to the SAME power amp channel.

Is it ok to use two preamps with the Anthem MCA 325 amp in this configuration?

Not an good idea to connect both RCA and XLR input into a power amp channel without a selector switch. It is possibility to damage the power amp input stage or preamps output stages or both!

What you need to do is connect the Front L and R preamp output of the Denon receiver to the Anthem STR preamp input and set that input to allow HT bypass, using xlr or rca output from the str preamp to the left/right channels on the amp.

For HT playback, turn on the STR preamp and select the HT bypass input.

I am getting some faint buzz from the center channel, but can only hear it about a foot from the tweeter at ear level.

One thing you can try to eliminate noise from center channel is insert a Jensen Transformers PI-RX

https://www.cs1.net/products/jensen_transformers/PI-RX.htm