To get the powertubes really cooking you REALLY have to push that master volume, even wallquaking volumes aren't gonna produce any powertube distortion, only very seriously high levels will do that. I have an Engl Fireball and have pushed it to 11 o clock(FUCKIN LOUD) and didn't feel any powertube breakup, I only tried it at around 2-3 o clock(got neighbor complaints from 3 stories below me) and it was a fuckin Sunday at 2 pm) and I STILL didn't get any powertube breakup, you need to push it much more. (funniest thing is that I bullshited them into thinking I was an audio engineer student and I had to finish my amplifier-project for college, the suckers fell for it but I only played for a few more minutes at 1 o clock.
Since everyone here has high gain amps noone notices that 99% of their sound is coming from their preamp, not poweramp tubes. The real way to hear how loud you have to push an amp to get powertube breakup:
Go try a Marshall JMP or any other 70's or early 80's rock amp, and see if you can crank it til you can get acceptable metal sounds, if the store owner lets you!
True powertube breakup is extremely difficult to achieve, most apartment dwellers with 50W tube amps like me aren't going to be experiencing it at home anytime soon.
Or even easier, try an Epiphone valve junior, put it the volume up nearly on full to get some actual distorted sounds, that's how far you have to dime the volume to get powertube breakup.