Lower quality and softness is logical, you are going from 1920 x 1080 to 720 x 480, about 17% of the original resolution.
Export direct from Premiere using Adobe Media Encoder (File > Export > Media) and be sure to check the "Maximimum Render Quality" button at bottom of interface. Using MRQ can increase the quality of the downscaling from HD to SD. In Encore, use Import as Timeline and multi-select the .m2v and .wav files that you exported. Any chapter marks in Premiere will carry over to Encore as well using this method. You will need to manually set the encoding bitrate when using AME, where Dynamic Link automates that.
If you are unsure about data rates, let us know the length of the program.
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