If the HTIB accepts hirez audio over HDMI, then use the HDMI. You will likely have to have the player decode the formats, because a quick google seems to say that your system only has legacy decoders.
If it can't accept the audio over HDMI, then use the optical. Yes, now the player sends video via HDMI to TV, and the optical is used for sound. You will get slightly improved bitrate with DD, and double the bitrate with DTS. I'm talking bluray here. But, this is not the "best" available to you on the disc.
However, who cares, no offense. Those speakers are probably of such low quality, no offense, that you probably couldn't hear the difference anyways.