From the launch post for the Sub gen 3 - although this accords with what I have always understood to be the case:

The custom-designed drivers take over the lowest frequencies for the Sonos speaker or home theater setup that Sub is bonded with, freeing up the speaker for higher highs, while the Sub handles all of the lowest lows.

It’s all explained here, where it’s clear that a true crossover is operating: 

https://www.sonos.com/support/en-us/sonos-user-guide/index.html#t=sonos-user-guide%2Fsub%2Fsub.htm%23TOC_Sub_settingsbc-6

@g-r-c,
The setup mentioned will achieve what you want here, there is no bypass for the sub crossover filter in the Sonos app when connecting a Sub to an Amp. Only the mid/higher frequencies above the crossover goto your connected passive speakers. 

In fact some users want the opposite to your requirement here and want the possibility to let the front speakers always play the full frequency range when a subwoofer is connected. So some often ask for a toggle-button in the App for whether the crossover should be applied for both speakers and sub or to send the full frequency range to both speakers and sub output and ‘perhaps’ let the control of the crossover be handled on the sub itself (if supported).

Those that want the opposite (full frequency range), can achieve it of course by connecting speaker cable from the Amp’s speaker terminals to a 3rd party subwoofer’s high level connectors. But then the possibility is lost to independently control the sub-volume level from the App.

Anyhow the Amp setup with the Sub (gen3) should be fine for what you’re hoping to achieve.

Keep in mind that part of what you’re paying for with Sonos is the wireless connectivity and aesthetics. If you don’t mind an RCA cable and the often uninspiring industrial design, you can likely get equal or better results with a third-party sub at lower cost.

For what it’s worth, I was never able to satisfactorily mate a Sonos sub with the (admittedly difficult to match) speakers I use with my Amp and ultimately switched it out for a third-party model. Can’t complain though as that gave me a good excuse to move the Sonos Sub over to a stereo pair of Ones, where it performs beautifully.

Thanks All :)

Thanks Ken - that’s clear

Thanks Ratty - I read that, but agree on close reading - it’s implied that the loudspeakers benefit from the signal to them being carved up (although the focus is more on the actual Sub) - it does say that if the main speakers are distorting, then the suggestion is to set the controls for smaller speakers (i.e. higher transition frequency on the high-pass-filter). Doing that wouldn’t have any effect if the Amp is always outputting full-range to the main speakers, regardless of setting - i.e. it does suggest that the frequencies to the main speakers are high-passed according to the control setting selected

Ditto John - it’s suggestive (but I prefer to know for sure)

Hi Early Rejecter - point taken, but my priority is the sound - nice to have the controllability and (relatively) funky looks (although I’d prefer to bury a Sub behind or by a sofa), but to me it’s not worth the expenditure on the Sub if it’s not helping an appreciable amount with clarity as well as extension.

BTW are any of you folks agents or employees of Sonos (I just noticed the impressive mammoth reply counts )

No one here in this thread works for Sonos, just enthusiasts mostly and trying our level best to assist other users.

The Sonos Staff/Employees here in the community are always clearly marked - Here is an example screenshot of a Staff member, so as you can see it’s easy to identify those who actually work for Sonos:

That’s what I was getting at — if the sound is your priority, there’s a good chance you could achieve equal or better results for less money. With a third-party sub you get the same high pass crossover vis the Amp, and (at least on iOS, don’t know about Android) the same sub adjustments that you get for the Sonos sub through the app. Not trying to discourage you from going with the Sonos; only bringing it up since you mentioned the cost. The one big advantage the Sonos does have is it’s profile makes it easier to place in the best position for the room; I’m more limited with the placement of the bulky box I replaced mine with — but in my case it’s for the best as the bulky box blends better with my speakers.