I don't see a spdif plugin for 1.x versions of foobar. So you will probably want to stick with foobar 0.9x (unless we figure out that you don't need a plugin in 1.x?)
Looking at the plugin website:
http://www.ga.cba.pl/it looks like it works via the file extension, so if you have some type of encoded autio (such as a .wav file that is DTS encoded) you'd have to rename it to .DTS to get it to play our the DTS.
I'm not sure we're on the same page about sound card settings. Are you changing your sound card sample rate from 44.1 to 48 when you play a DVD files vs. CD files? As far as I know that doesn't happen for you in foobar or any other software (although some software can be set to resample).
For playing files from a DVD (48 khz) please verify that your sound card is set for 48Khz. Also, there can't be anything in the audio chain that is changing the volume. That's probably part of what the foobar plugin does, but if you were using a generic software player and SPDIF output, you would need to set the volume at max, disable any EQ, etc. etc. to get bit perfect playback so your HT will recognize and decode it.