I have a NAD T752, a NAD T513 DVD, a Comcast HD cable box, and Dynaudio Audience speakers (86 sensitivity @ 4 ohms). On the T752 setup, I have speakers set to small, bass crossover set to 60, and I have the channel adjusts set to +12.
When I watch regular TV, I get a good listening sound at -40db. When I watch HBO or a digital channel I have to increase the volume to -25db to get a good listening sound. When I watch a DVD, I have to increase the volume to -15 or -20 to get a good sound.
Would adding an external amp change any of the above?
As long as you can play as loud as you want without clipping the NAD's amp, there is little to no benefit to be derived from buying more amplification.
I would say to borrow a friend's or dealers amp to see if it makes an audible difference. But I would attach it to a switcher and have a friend do the switching without your knowledge. That way you remove the very strong psychological component that will tell you--"The amp must be making this sound better". You put on your favorite cd and dvd and have about 20 or more switches performed. If there is little to no statistical difference---don't get the amp.
But unless you are bothered by the current state of affairs, why bother?
Adding anamplifier is not the solution. You have found the solution. Turn the volume up. This is a function of input levels. Though it would be nice if everything had the same output level and impedance it doesn't work that way in real life. That's why some expensive pieces of equipment have trim pots to balance the levels on all inputs. Think of it like a faucet. If you have lots of water pressure coming to the faucet you don't have to open it much to get lots of water. If you have less pressure to the faucet you will have to turn it open more to get the same amount of water. The different volume levels are the water pressure to the faucet. It is not unusual for different sources to have somewhat different output levels and therefore you have to turn the faucet open more to get the same amount of water. If by regular channels you mean not through your cable box this is very likely. The difference between the cable box and the DVD is usual and if you put a different DVD player in the system the problem might stay the same, get somewhat better or somewhat worse. Adding an amp is not the answer, besides, where would you put it in the system? Just remember to turn down the volume before you switch between inputs and find something more important to worry about.