Just a reminder but I'm sure you know your center channel speaker should always match your front speakers. So if you change brands for your fronts that means dumping more money on a new center speaker.
I do have to agree with James63 comment as the center channel in a HT application does most of the dialogue. I'm not familiar with your Marantz receiver but in the past as far as an A/V receiver Marantz were also good in a two channel application. I had their top model receiver from the late 90's, can't remember the model but it was gold, and it was great. I ended up using two Aragon mono blocks for my two channel. Unless you move more towards two channel than the Diamonds may be overkill. Maybe upgrading to B&W's top model center channel would be a better bang for your buck at the moment. Good luck
I do have to agree with James63 comment as the center channel in a HT application does most of the dialogue. I'm not familiar with your Marantz receiver but in the past as far as an A/V receiver Marantz were also good in a two channel application. I had their top model receiver from the late 90's, can't remember the model but it was gold, and it was great. I ended up using two Aragon mono blocks for my two channel. Unless you move more towards two channel than the Diamonds may be overkill. Maybe upgrading to B&W's top model center channel would be a better bang for your buck at the moment. Good luck