Amplifier Power tubes - important to sound?

Yes very important also important is coupling capacitor(s) for cleaner sound quality of the amp

Right out of the Mc 901 manual:  

The MC901 will function optimally with any correct tube type that is functioning within the design specifications of the MC901. Only when a Tube falls outside of the acceptable range will any diminishing of performance be heard. Because McIntosh Tube Amplifiers are engineered to use Tubes in a way that does not rely on the Tube’s individual character and can perform perfectly provided the specifications are met, there is no advantage to seeking out expensive esoteric tubes.

"Because McIntosh Tube Amplifiers are engineered to use Tubes in a way that does not rely on the Tube’s individual character and can perform perfectly provided the specifications are met, there is no advantage to seeking out expensive esoteric tubes"

I don't have these amps but he already stated that he changed the signal tubes and the sound improved , aka the character of the tubes came through, so why wouldn’t that also occur with the power tubes? I’m not aware of any tube amp, or other tube component where the character of the tubes doesn’t come through. I presently have 3 tube integrated amps, had a couple of others previously, and have a tube DAC  whether it's the signal tubes, power tubes or rectifiers they all impact the sound in the respective  sound. All their statement reads to me is marketing hype.

Or a way to justify them installing mediocre tubes with their amplifiers. Somehow commentary in manual does them less good service because having the ability to modify sound using different tubes is an important feature and why someone buys a tube amplifier.

What did they do that prevents a tube character from coming through? Is this a good thing?

Well that is a perplexing concept… engineered in a way to prevent the tube character coming through. The only way I can think of doing that is to just power them but not put them in the audio circuitry… so, make them for show only.