Is a preamp necessary?

Sometimes it works, sometimes it doesn't. Not all DVD/CD players have enough output to drive an amp to full power. Conversely, some amps have comparatively low gain and don't work well even with moderately high output CD players.
The only way to find out is to try. Just make sure you rig a temporary volume control. It doesn't have to be super high quality--just enough so that you can turn it down should it be too loud.

I agree with Tobbe. Most high quality CD players can drive them, but make sure there is a volume control. If you dont plan on using a turntable, headphones, or other device, and dont need the gain, then a preamp is just another device in the signal path and can be done without. But the only way you can know for sure is to try it!