Can I ask for advice on the best way to connect the MD600 and 818 if I want them in separate rooms and don't want for practical reasons a long length of cable between them?
The consensus from older threads I've been able to find seems to be that a plug-in wifi extender to Ethernet cable would be better than units that send data over the electrical wiring. Is that still the case, and is there a particular product that is generally believed to be best?
As Chris says, Sooloos is designed to be hard wired. The Sooloos audio protocol is based around the less reliable UDP protocol not TCP so any any network glitch will require Sooloos to do more work using a fairly basic error recover mechanism. Does this affect audio quality, I do not know.[fixtypo /]
So anything other than a true hard wired connection is a compromise. If you want 100% audio quality, you want hard wired.
Of the alternatives, powerline and wireless are the most obvious candidates. There are others, such as network connection similar to powerline but over coax, ie aerial/sat cable. You may have spare sat/aerial connectors in the wall, which may be suitable to use - I guess they must not be isolated sockets, but not sure.
Out of wireless/powerline, I would not trust wireless to anything important, but have been impressed by the robustness of powerline. One HHer ran his core (the most network dependant Sooloos component) connected to powerline and I think it worked without issue. However, when he moved to a different store/core, an audio improvement was apparently heard. Who knows whether this was network, hardware or placebo.
Personally I don't run anything important over anything but a hard wired connection. But my interconnection is via powerline and in the 6+ months that I have owned it, it has been nothing other than totally reliable. I have 4 powerline adaptors, two of which have mains passthrough AND mains filtering. These are notably faster endpoints than my non filtered ones.
As fore wireless, before I went to powerline, my wireless was influenced by lots of things - mostly environmental outside my control. I tried boosters and using my Sonos mesh network, but never achieved total reliability that I expected with a hardwired connection. But then I live in a long bungalow with proper brick walls (probably made with slag/iron from old furnaces) which is not ideal wireless environment.
At end of the day, it comes down to how much of a compromise you can tolerate and what your environmental factors are.
Personally, you have an 818, its not worth compromising.