I cant tell you for sure what exactly the difference sounds like because when I ordered my set from eurotubes,, believe it or not, Bob wound up talking me out of getting the high gain preamp 12ax7's.
He asked me what I wanted and I told him,, high gain, and he asked me do I ever run my current set up with the gain maxed 100%? I had a feeling I knew where this was going to go so I almost lied but I told the truth and said no I dont usually run my gain 100%.
He gave me an example what the difference would be,, he said that if I ran my setup at full 100% on 10, It would be like I can go to 11. That was the difference. Anyways if you order from him,, talk to him get some ideas and if high gain is what you want,, deffinatly get them.
Honestly though I believe you would do much better for yourself if you called dougs tubes,, I must easily have 3 or 4 sets of different brand powertubes from dougs which are better than JJ'S. If your set on trying some JJ'S just get the powertubes,,, try some tung-sol's or Penta Lab 12ax7's instead. Much better to me.
Good luck. By the way I have never ordered any preamp tubes with the high gain option but on the other hand they offer these for some reason,,, I really wonder sometimes if its an extra couple of bucks in their pockets from those people who insist on everything being the highest gain possible.