DTS-MA = lossless compression
PCM = uncompressed
DTS-MA and TrueHD are used in favor of PCM on media because they reduce the amount of space needed to store the soundtrack on the disc without any loss in audio quality, allowing producers to use that space for higher quality video and/or more content.
When you set your player to send Multi channel PCM, your player decodes the compressed audio and sends the uncompressed bits to your AVR. With bitstream, the AVR decodes the compressed signal encoded on the disc and plays back the uncompressed bits. So in either case it should be the same bits getting sent to your speakers.