Well, as we discussed earlier, the Dolby Digital sound has a greater dynamic range than Pro Logic, so when you were watching DVDs on your TV, you would have only had the 'flatter' Pro Logic sound. With Dolby Digital, you will notice that the quiet bits seem quieter, and the loud bits MUCH LOUDER!!! With that in mind, I wouldn't advise having the amp at full volume, as you may even damage it or your speakers, if the sound volume suddenly increases during a movie.
That's the only explanation I can come up with for the lack of volume, but of course, there may be an underlying technical problem.