That said, if an amp or DVD player says Dolby Digital (or Dolby Digital 5.1) it will be able to deliver up to and including Dolby Digital 5.1.
The only difference in types of Dolby Digital hardware is whether it can deliver Dolby Digital-EX or not. If something is capable of that (6.1) then it will say so, if it doesn't then it can't.
I'm pretty sure that's right anyway.