Two Preamps Into 3 Channel Amp

Two answers: electrically yes, its okay. Worst that may happen is you pick up some ground loop hum. Which might be bad or might be zero but in any case won’t break anything. But sonically no, its never okay to have a AVR connected to a stereo. The whole purpose of HT is artifice. Show. Glitz. Artificiality on steroids. The whole purpose of stereo is quality. Substance. Palpable you-are-there-ness. The two are at odds.

Connecting AVR and stereo therefore is never a good idea, other than as a temporary stepping stone to eliminating the AVR and all its multi-channel crony cabal components. Except for the subs. Those you can keep.

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 assume then that the best option would be to use the receiver to power the center channel then and just use the left/ right inputs on the amp from the str preamp? 

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.

The denon receiver only has rca preouts so that currently goes into the center channel on the mca 325 amp. The left and right preouts of the receiver are going to the ht input on the anthem str preamp. Im using the xlr outputs from the left and right str preamp outputs to left and right xlr inputs on the mca 325 amp.  So I am using the rca input on the center channel of the amp and the xlr inputs for left and right channels of the amp.  I am getting some faint buzz from the center channel, but can only hear it about a foot from the tweeter at ear level.