Did some more research and unfortunately I think you're right. Neither the Sonos Connect nor the Sonos Port seems to be able to link with the Sub alone. Strangely, the Sonos Amp can do this. But since Beo-speakers are active/powered you can't connect them to Sonos Amp.....

Is it possible to connect the Beolab 18 with non-Beo speakers? And how?

Did some more research and unfortunately I think you're right. Neither the Sonos Connect nor the Sonos Port seems to be able to link with the Sub alone. Strangely, the Sonos Amp can do this. But since Beo-speakers are active/powered you can't connect them to Sonos Amp.....

Is it possible to connect the Beolab 18 with non-Beo speakers? And how?

Hi,

You can link the Beolab speakers to a Sonos Amp by adding this converter to let you link the Sonos Sub wirelessly: 

I'm open for suggestions... Didn't want a sub in the first place. The obvious option is of course the Beolab 19. But I could consider others as well...

Maybe get simple RCA y-splitters for the output of the Sonos Connect, and send one side to the BeoLab 18s and the other side to "any" powered subwoofer, which you will hide away?   Most powered subs will filter out the high frequencies, but this method requires a sub with stereo line-level inputs, also a crossover frequency dial on the back, because you'll want to adjust its frequency range versus the Beolab 18's anyway.  You'll turn the volume up slightly in the app to compensate for the split.  (Decent but inexpensive subwoofers would be Rythmik, or SVS, or Hsu.  Don't remember which models have dual line-in instead of a single LFE jack, but plenty.  Just not the Sonos Sub (wireless only), nor the Beolab 19 (Powerlink, not line-in triggered IIRC)!) [Edit: Oops, "inexpensive" subwoofer depends on where you live: subwoofer shipping is costly and "free returns" aren't, if there's no in-country distributor.]

Thanks for all your replies!

I tried connecting my Beolab 18 speakers to an active subwoofer 

1) Yes.

2) Likely something is poorly grounded in the audio path: This means the shielding of the audio cables (or wires internally), somewhere in the audio path, is cut or loose.  Yes, it is also possible that the shield is OK but you just happen to run the cable parallel to AC wires for some really big electrical equipment.

To test #2, purchase two little female-to-female RCA connectors, and take the sub out of the circuit without moving anything.  (Alternatively, unplugging the sub's AC power -- since its RCA connectors are simple pass-thru, no filters or anything -- should be the same test, unless you have some noisy piece of electrical equipment right near it.)  I.e. "differential diagnosis": is the hum with the cables, but without the sub, still there?

3) I would say "no that's not normal" but I don't own 18s to know how immune they are to electrical noise.  The easiest way to know for sure is to go to your dealer on a quiet afternoon and listen to theirs.  Despite having spent a lot of time on setup, theirs will be an electrically-noisy environment with all the gear, so whatever they have for noise should be the upper limit on what you should expect at home.

To test #3, purchase two little RCA "grounding" or "shorting" plugs (or make your own) which are just a wire from the signal pin to the shield.  Plug 'em in to the 18s -- after triggering the line-in power-on of course -- and see if the little buzz goes away.  Again, differential diagnosis: is it the speakers (their amp board, actually) or the cables (or their source) causing it?

P.S.  That Gallo sub has the lowest W.A.F. of any I have ever seen -- even worse than the cylindrical columns from SVS! 😁 So I can see why you might have run extra-long cables to keep it well out of the way!  Longer cables pick up more noise, of course, if there is any in the first place.  But if the Gallo sub is responsible for the hum (#2), it is possible that the internal wires are un-shielded and its power supply transformer is radiating right beside them; this would be an engineering failure.  More likely would be a manufacturing glitch; connections to the outside world are still done by hand many places.  An easy fix is to put Y-connectors right at the L/R RCA jacks on the Sonos, and run *separate* cables to the sub and to the 18s, not using the Gallo pass-thru jacks.  That doesn't prevent radiating the 60 Hz hum back down the cable, all the way out to the speakers, but so much noise would have to be a truly egregious fault (and cause for a return).

Thank you for your thorough answer. Will try this as soon as I can. 

Are there any benefits of letting the signal pass through the subwoofer - or disadvantages using a Y-cable directly from Sonos? 

According to the Gallo manual there seems to be different ways of connecting the speakers with filters at certain frequencies, depending on which connection you use (RCA vs “loose wires”) but I don’t really get it... am I getting it wrong? 

PS I kind of like my Gallo Sub. Actually find the Beolab 19 strange looking... But it’s all a matter of taste :) 

Realised the link to the manual was wrong... this is actually the subwoofer I have. Different looking connections at the rear (but otherwise the same WAF ;-)

https://galloacoustics.com/wp-content/uploads/2019/08/tr-1-manual.pdf