It's a matter of speaker setup (a.k.a. bass management), which you'll have to configure in two places. DTS and Dolby Digital are passed digitally to your receiver (probably, anyway), so set your receiver's speaker configuration for two channels only (front left and front right, both large, everything else off). This will down-mix 5.1-channel programs to 2.0.
For DVD-A, do the same thing in your DVD player's speaker configuration. (You probably know that DVD-A is output strictly as analog, so DVD-A will have to be processed in the player.) Since your DVD-A programs will be down-mixed to two channels, you can simply plug the output into a two-channel input.
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