What equipment do I need to use with an HD DVD player to get the proper sound?

Ok, I am a big home theater buff, and just finally getting into the HD players. I am ready to buy an HD player but am so confused on how the audio works. To get the dolby digital plus of dolby tru, does the audio need to be played through the HDMI cord and into the AV reciever? Heres my setup: 7.1 Surround, 50 inch plasma with 1 HDMI input, and Pioneer THX reciever ( but with NO HDMI input) So my question is, If i buy an HD DVD player, and just run the HDMI cable from the dvd player to the tv ( since my reciever doesnt have an HDMI input), will I only get the HD pic, and not the awesome sound features like DD+? Basically, can I get the great sound HD DVD players have to offer by running an optical cable to my reciever, or does that sound only come through the HDMI? So confused :) Thank you!