When playing two channel music in Dolby Surround, or in one of the Pro logic iterations in older models, vocals can be too center focused as they are processed. This sounds strange compared to the two channel presentation. Center Spread simply, well, spreads the vocal out evenly across the front stage and the surround channels handle other musical sounds. It really shouldn't be used for anything other than two channel music with vocals. Though, there are some bad multichannel music mixes that can benefit from it as well as the vocals are center focused in the remix. Different receivers will have different limitations with Center Spread as some aren't allowed to apply Center Spread to an Atmos track.
The Denon S760H is underrated and is a very capable little receiver. I almost kept mine but HEOS didn't do it for me. I agree about the crossover settings. I'd set them at 100Hz or 120Hz for all channels as well with the sub LPF of LFE set at 120Hz. You might try the sub's gain knob at 50% and definitely set the Crossover knob at MAX(180Hz) so it does not interfere with the receiver's setting. It looks like room is an issue, but if you can raise the speakers and or stands, blocks perhaps, to clear the couch and move each away from the fireplace about a foot and turn them in just slightly, I think your center channel output will benefit from the increased distance from the fronts. Many run out and spend more money on power when they could have spent just a little time improving speaker placement and tinkering with settings to improve the sound altogether. Nice room by the way and good dog. Enjoy your setup!
