OK - Basic Audio 101:
The new larger Blu-ray discs can accomodate larger audio files - essentially the audio version of HD video. Studios are now putting uncompressed 5.1 PCM tracks on movie discs, exact copies of the studio masters, which are the best possible quality. Uncompressed tracks take up a great deal of space. So, Dolby Digital developed TrueHD and DTS developed Master Audio (dts-MA). TrueHD and dts-MA are "lossless" codecs. They compress the original files to save space during encoding, when they are put on the disc. The decoder in the player or receiver uncompresses them back to the original PCM. These are lossless codecs because no audio data is lost in the process.
Dolby Digital and DTS are older "lossy" codecs developed in the 1990s. Dolby had to really squeeze down the audio files to encode them on the actual film. DTS encoded the audio on a CD that rolled in sync with the film. CDs don't have much space either. DD and DTS are lossy codecs because they have to discard some of the audio data to squeeze the files down to rather small sizes. Quality suffers, as a result.
Optical cables (and the SPDIF protocol they use) lack the capacity for these larger audio files. Optical is limited to the older DD and DTS files and two channels of uncompressed PCM. If you want to send lossless, multichannel digital audio to your receiver, you have to use HDMI. If you try to play TrueHD over optical, the player will use a legacy DD track instead.
However, there's one other choice. If your Blu-ray player can decode TrueHD and/or dts-MA, then you can convert it to analog in the player and send it to the receiver over six separate analog cables, one for each channel. The receiver simply amplifies the signals and sends them to the speakers. Your new Blu-ray player can decode TrueHD, but not dts-MA. So, with analog cabling, you can get lossless audio from uncompressed PCM tracks and TrueHD tracks, but not from those encoded as dts-MA.
As for your old Pioneer player, use optical. It doesn't have the new, larger audio files and there's no advantage to analog cabling.