First thing first, I'm a fan of Marantz and have heard good things about Yamaha in general. While I liked the Onkyo I've owned in the past (RZ900), I haven't kept up with them lately. If you plan on using room correction, Audyssey XT32 would be best (with the app, so you can limit EQ) which comes equipped on certain Marantz and Denon models.
You might already know the below rambling, but ultimately get the amp and specs you need, without worrying about sound signature. I personally believe the warm/bright/forward/airy adjectives people use for amps are snake oil placebo.
For watts per channel...few things to consider. How much power you need will depend on several factors, including speaker's sensitivity, room size, distance to listening position, your volume preferences, content listening, headroom etc.
For most normal conditions, especially with 2 channel music 75wpc is plenty. However for the specs make sure they're rated for 20hz-20khz with 2 channels driven and distortion like .08% or .1%. Some manufacturers fudge the numbers and don't show the distortion or only drive with a 1khz signal etc.