Hi there,
I’ve done many tests and to my surprise the PowerLink connection sounded more natural to my ears. Over optical they sounded a bit bright and digital. Also you will loose around -3db volume output when using optical, but that’s not a big deal.
After reading Geoff Martin’s blog I realized the amps in the 18s are always running at full power. That’s a less than ideal place for a digital signal to enter.
For PowerLink, I’ve used a high quality CAT8 cable made by the Swedish company Supra Cables. They are a well respected manufacturer in the audiophile community. The wiring is the standard T568B but very important is to connect the cable’s shield on both ends (under the gold plated metal caps). This way there’s absolute no static noise.
If you do go for optical, don’t spend too much money deciding. There are 2 types: normal ones and the very expensive quartz core ones. QED (UK company) makes an affordable quartz core cable, if you can call it that at 80€/meter. The others go for 3-500€/meter.
Now some technical remarks about PowerLink vs. Optical SQ.
My guess is that the Signal to Noise ratio is better over PowerLink than Optical.
I also suspect the DAC in the BeoSound Core to be better than the DAC in the Beolab 18.
Because the signal must be Volume Regulated, mathematically you will loose some bits of information at any volume other than maximum volume over optical.
They say you won’t hear a difference at normal listening volumes, but trust me it will affect the interpolation in the final Digital->Analog conversion. I will not get into details about DACs bit-depths and recordings dynamic ranges :).
Hope this helps. I also wired my Core to Ethernet to avoid any WiFi interference.