Best Connections to Older AVR
Normally, I don't recommend the connection scheme specified by the guy at the store. I usually recommend connecting the HDMI outputs from your source devices (Blu-ray player, cable box) to the AVR and the HDMI output from the AVR to the TV. As you point out, however, the Cambridge 540R V3 does not decode audio via HDMI; instead, it requires audio from a separate coax or Toslink connection, which means you would have to connect one of these outputs from the Blu-ray player and cable box as well as HDMI. If your Blu-ray player had multichannel analog outputs, you could use those instead, but the DMP-BDT210's has only a L/R stereo pair of analog outputs.
Since the 540R V3 must get audio via coax or Toslink, there is no performance difference between the two connection schemes, so it really doesn't matter which you use. If you connect the Blu-ray player and cable box to the TV via HDMI and the TV's optical output to the AVR, you'll be using the TV to switch sources; if you connect the source components to the 540R with HDMI and Toslink audio, you'll be using the AVR to switch sources.
The best choice for you depends on which way you find easier to use when selecting which source you want to watch. Another factor is where the components are physically located. If the Blu-ray player and cable box are near the AVRsay, in an equipment rackand relatively far from the TV, it's more convenient to use short cables from the source devices to the AVR and a single longer HDMI cable from the AVR to the TV. But if all the components are near the TV, this consideration is unimportant.
Update: According to WC in the comments below, most TVs limit their optical digital-audio output to 2-channel stereo. I never use that output, so I don't know if it's true in most cases, but if it's true for the ST30, your best option for multichannel audio is to run HDMI and Toslink from the Blu-ray player and cable box to the AVR and HDMI from the AVR to the TV.
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