my denon can pass test tones through each speaker.
I can do it from the remote.
maybe the next time yours fades out, you can quickly put the thing in test mode, and see if the center channel is fading.

Of course you could just get an axiom center speaker, it couldnt hurt

Axiom M80, Ep600, Qs8, VP150, Crown XLI 2500 , DBX Driverack PA2, Focusrite Scarlet 2i4