You only need one decoder. Most people pass the digital signal to the prepro. The one in the receiver/prepro typically has more features, flexibility with bass management, and perhaps higher quality.
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The first response from Inscrutable is the CORRECT one. Get a DVD player that also has DTS capability (most do now) but WITHOUT a decoder (for DD/DTS), and let the pre-pro decode the DD/DTS signal. You can get an excellent player now for as little as $225 retail (and that even gets you Progressive Scan), at Bust Buy and other stores. Also, The Secrets of Home Theatre site is an excellent source for info on DVD players.
There's a difference between being able to output the signal, and being able to decode it. It's best to leave the decoding to a processor, however the dvd player much be able to support (i.e. spit out) the dts or dd format. most dvd players these days can do that, and usually say so right on the faceplate or box (little dolby digital and dts logos).