QuoteI've also read that power amps basically do NOT change the sound at all, they just amplify the sound.
This entirely depends on how the power amps in question are designed.
I can tell you for instance that power amps in amps like Vox Valvetronix series or Peavey TransTube & Vypyr series do far more than amplify sound.
Most guitar amps do at least have current feedback, which at least makes the frequency response VERY non-flat. Advanced designs like Peavey's, Pritchard's, Quiolter's, etc. not only do that but may also soft clip, compress, hum and crossover distort like many generic class-AB tube amps.
Saying that power amps only amplify and nothing else is blatantly ignoring several decades of design dedicated to emulating tube amp behaviour with SS.