I am starting calm birth classes soon, and used a TENS last birth, but haven't done both together. BUT I just wanted to say that the TENS is a distraction as you keep having to adjust it, so Im not sure it would go that well with what you've learnt. Saying that though, it is probably worth hiring a TENS and taking it anyway, you might want a break and they are so cheap to hire and easy to use, it wouldn't matter if it didn't work out.
Good luck xo