TrueHD doesn't pass through optical, so it is being downsampled to DTS. Your best bet is to set the player to bitstream, and let the receiver do the audio decoding and not select HD audio in the disc menu.
I have a BD-1600 Bluray player connected to a PS58B850 via HDMI for video and to a Yamaha RXV-3200 via optical for Audio. This receiver has no HDMI capability nor support for the newer sound formats. The issue I have is that Blu-Ray disks with Dolby TrueHD soundtracks are converted inside the player to a format that is recognised by the receiver as DTS. The resultant sound is "wrong". The level changes constantly, the Centre channel is totally screwed with most dialogue being unintelligible. When a PS3 is substituted the sound improves dramatically (the connections are the same). The PS3 does not do any "conversion" - I think it just downmixes Dolby TrueHD to DD 5.1 and DTS-MA to DTS. The BD-1600 has the latest firmware.