With the above, the .1 effects channel isn't available to be used for any sub; for that one needs to have a Sonos Arc/Beam kind of product if the Sonos Sub is to be used for .1 effects.

Kumar is correct. LFE is not defined for a simple two channel amplifier. In what we usually classify as an ‘A/V’ or ‘Surround” capable unit, the LFE channel (if present in the source material) can be routed to a subwoofer output or the front L/R if there is no subwoofer. Often, one can offload bass to a subwoofer at an adjustable crossover point.

Thanks for the quick answers.

https://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/38_LFE.pdf

I understand from the text that LFE is not the same as Subwoofer, but can contain additional information low freq information that is not presented to the subwoofer.

My DM605-S2 active speakers have special LFE inputs I would like to use, but I understand that this is unfortunately not possible with the Sonos and it seem with allot of amplifiers.

Thanks for the time, if someone has some advice I would be curious to hear.

What happens to the LFE channel, if present at the source, depends on the amplification system. DOLBY or DTS sources my have exclusive LFE information. A stereo mix may include the LFE content and this can be routed to a subwoofer.

Looking at the speaker specs, it is a strange product that I haven't come across before, one that incorporates an active subwoofer within floor standing passive tower speaker pair cabinets.

But for sure, the LFE inputs need to be driven by the LFE out on a AV processor, the manual is clear about that. Every HT(AV) amp has these outputs, no stereo only amp does. As far as I know.

How this works in practice, especially for music, is questionable at first glance  - if the AV receiver is set to stereo mode as usually works best for music, there would be no signal going to the sub units, and how would the music sound at the low end when deprived of these? 

The other question is when the AVR is set to 5.1, how much would the presence of powerful sub units inside the same enclosure affect the mid range delivery?

As my speakers take the full low range from the L/R channel I am oke for the subwoofer part, also to listen music. The LFE is only used when available as a additional channel in the signal and the mixed in.

Thats why my question for the Sonos Amp is it a LFE, LFE+Main or just a Main with filter. As you have HDMI ARC input they could have put the option like an A/V receivers have (LFE or LFE+Main). But it seam as Sonos is only 2.0 and not 2.1 the subwoofer output is a 80hz filter. 
For now I am looking to a e.g. Sony receiver like the STR-DH590 and just ise it as Stereo with the LFE. 
thanks to have taken the time to look with me. 

To share my final result, maye it could help someone else or Sonos to upgrade the software.

I finally bought the Denon AVR-S650H. You can choose your speakers so you can make it a stereo with subwoofer (2.1) and the sub is changeable between LFE and LFE+main. It is also not too big and affordable.