"who has my DBT's?"
In all seriousness fellas, you are really digging a deep one here.
Yamaha is not bright, Marantz and NAD are not warm, Rotel isn't mostly neutral but a tad bright. The acoustic differences among amplifiers based on the same topologies are neutral across the audible spectrum unless they are 50 years old or made by "Theater Research".
Now the new digital amplifiers do sound different, but not many have delved into that for audio. Rotel does have a class D power amplifier available, and is quite powerful for its relative size, but it lacks grunt for low impedances (tough loads).
Now this isn't to say all class B or class A/B amplifiers sound the same with certain speakers, because they don't. Pairing 4 ohm speakers with 87 dB @ 1watt/meter with a low level receiver is not going to produce good audio playback in a sound system. Bigger and more stable amplifiers will handle the loads presented by more difficult to drive speaker systems.
If a sound system sounds to bright or edgy on the highs it could be a few problems. A. would be the speakers themselves which carry the largest effect on the SQ of any sound system, B. would be the room's acoustic properties that can be adjusted with acoustic treatments such as diffusers and acoustic panels, C. would be the source material, the selection of movies or music effect the SQ, not every recording is the best and there isn't much you can do to correct it with most music and movies as your options will be limited, D. would be the amplification. If the amplifier is not equiped to handle the demands of the load (speakers), the system will not perform as was intended.
If the system sounds bright to you perhaps the speakers are the source of the brightness, or room acoustics. Careful selection in the material you will listen to for fidelity can improve those SQ factors, and lastly, but not least ample power for the speakers in the room can have an affect even at medium to lower levels of output.
OP, what speakers do you use with your Yamaha receiver? Also what model is the Yamaha receiver you are presently using?