The difference is the gain where a 12ax7 = 100, a 5751 is 70 and so on. I often recommend for instance that if you want more headroom choose a 5751 as a replacement for a 12ax7 with a black plate triple mica being some the smoothest and warmest tone. Guitar players who have distortion pedals can substitue a 12au7 due to its lower gain and it makes the overdrive much more usable. If you look online you should be able to find a chart of the difference in gain for all of these family of tubes. Joshua
