It entirely depends what sort of amplifier/receiver you are connecting to;
If your amp/receiver can handle DSD or PCM over HDMI, then this is the simplest way to connect. This way, the 650BD/651BD/751BD/752BD will simply be acting as a transport and the amp will be doing the decoding, so the audio quality will depend largely on the quality of the amp/receiver.
If your amp/receiver can't handle HDMI audio, or you want the 650BD/651BD/751BD/752BD to do the decoding (so that the sound quality is more the result of the Blu-ray player than the amp) then you can connect the analogue multi-channel outputs into the amplifiers multi-channel direct input.
You will only get the benefit of the 751BD/752BD's Wolfson DACs and ATF upsampling by using the analogue multi-channel outputs.
The 650BD, 651BD, 751BD and 752BD will output DSD over HDMI if your receiver will accept it but also output multichannel PCM that can be received by the 650R, 351R, 551R, 651R and 751R.