Any chance you can swap the Port for another device to see if it also has volume issues on that input? 

There may be too much information, I’m confused.

From what I’m reading, you have your Sonos Port connected to the Cambridge Amp, which is then feeding the Kef speakers. Is that correct? What is the source you’re feeding through the Sonos Port? What exactly are you adjusting the Oort’s output for Variable or Fixed on? Fixed would technically be pass through.

I thought KEF’s were self powered, why are you feeding the Sonos output through a Cambridge Amp?

I think I’m not understanding your setup at all, and mangling it. A quick graphic on how things are hooked up might help.

Sorry if it’s a bit confusing:

1. The pair of KEF’s are passive, powered by the Cambridge amp.
2. No line in source to Port. I group it wirelessly through the Sonos app with my existing sonos system.

Sorry I don’t have any pics at the moment but let me try to break it down:

Two separate set-ups:

1. Arc + pair of One SL + Sub (“Living Room” on Sonos App)
2. Pair of passive KEF’s, powered by cambridge evo 75
3. Port listed separately as “Living Room 2” on Sonos App

Amp and the port connected via RCA cable. I’ve got everything working together by grouping “Living Room” and “Living Room 2” on Sonos App.

Thanks, that does help Pictures aren’t necessary!

So, you’re grouping the Arc’s input (from HDMI-ARC) with the Sonos Port, by grouping the two rooms together, and ignoring the 75ms delay between the two. The Port is supposed to be feeding a line level output to the Cambridge, which then is sending the amplified signal to the KEFs?

It’s hard to rule out an issue with the Port, but I’d be willing to bet that the issue is more likely to be the cabling between the Sonos Port and the Cambridge amplifier. You could easily (and possibly, depending on what equipment you have laying around) by swapping the Port with some other device that has a line level output, like a DVD player, or tape deck, and see if the volume changes on the KEFs. Then switch back in the Sonos Port, and double check that the RCA cables are properly seated in the Port’s line out jacks.

If that doesn’t change anything, I’d submit a system diagnostic within 10 minutes of recreating this playback problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.