I've found that using different brand speakers sounds okay but it is obviously better to try and match all the front speakers up. Differences between the front speakers tend to show up if you've got a scene where the speech interchanges from the left to right to centre speakers. Any tonal differences will also show up when you play test tones through the speakers.
In my experience, the different amplifiers (40 watts to 90 watts, <0.5 % thd) I've used sound fairly indistinguishable from each other, though I never put my system up to deafening volume levels. Some speakers are meant to be more electronically demanding, like Quad speakers or Bowers & Wilkins speakers, and might require more of the amplifier. Still, I'd bet you'd need to turn the volume up very high to notice this.