Is the Sub you’re using self powered? The Sonos Amp doesn’t power its Sub output, it is line level only. It is designed for use with self powered Subs, and doesn’t have any amplification to push a passive subwoofer.
You really should contact Sonos support, send them a diagnostic so they can see the internal data on the Sub connection and why the App is not showing level controls.
I did, and they did absolutely nothing. In fact, they blamed it on the subwoofer... Even though it's obvious the app isn't working properly and is blocking a feature of the amp.
I did, and they did absolutely nothing. In fact, they blamed it on the subwoofer... Even though it's obvious the app isn't working properly and is blocking a feature of the amp.
Please explain how this is obvious?
What is the make and model of the sub you are using?
I did, and they did absolutely nothing. In fact, they blamed it on the subwoofer... Even though it's obvious the app isn't working properly and is blocking a feature of the amp.
Please explain how this is obvious?
When something is plugged into the RCA port, the amps are meant to bring up a subwoofer control menu. They do not.
What is the make and model of the sub you are using?
I've tried a couple, JBL ES150P and a KEF Kube sub. Both work fine with my Onkyo AVR.
I've also tried multiple RCA cables including the belkin one I use with my AVR. Same result each time, no sub controls in the amp and a super low output.
Certainly no shortage of power in those subs 
Unfortunately I don’t have an Amp I can test with anymore, but it sounds like something isn’t right with what happens after the rca auto detect should occur.
Off the top of my head, even ignoring the lack of sub controls in the app, I would expect the crossover to be applied by the amp, which I think used to default at 80Hz with the Sonos sub so you should definitely get some bass. If the crossover is actually at minimum, then there will be very little bass being sent to the sub.
With 3 amps all behaving the same it seems very unlikely to be an issue with the Amp hardware, but whether it is firmware or the app that initially sets the crossover I don’t know. The lack of sub controls in the app sounds like a bug to me, but again whether it is just an app bug or a firmware+app bug I couldn’t begin to guess.
Yeah, this is where Sonos really needs to take the wheel. If I push the gain to maximum on the subs, there's a bass signal, but that doesn't really solve the fact the output is so quiet I'm having to max out the sub gain. It's supposed to be an adjustable output for this reason.
So frustrating :(
Is the Sub you’re using self powered? The Sonos Amp doesn’t power its Sub output, it is line level only. It is designed for use with self powered Subs, and doesn’t have any amplification to push a passive subwoofer.
Yes, well aware of this. My original post mentions they have their own amplifiers with gain control.
Off the top of my head, even ignoring the lack of sub controls in the app, I would expect the crossover to be applied by the amp, which I think used to default at 80Hz with the Sonos sub so you should definitely get some bass.
Very odd “defect” for the quoted reason; What happens in the app for the Sonos Amp if the RCA wire is plugged only into the Sonos amp, and not connected at the other end to the Sub? A dead end probably, this trouble shooting step, but no harm in trying.
Another experiment - with the wire still not connected to the Sub, if you pull/insert one end of the RCA cable into the Sub out jack of the Amp while bass heavy music is playing, do you hear the bass go missing in the main speakers when the cable is inserted into the Sub out on the Amp? And reappear there after you pull it out?
Off the top of my head, even ignoring the lack of sub controls in the app, I would expect the crossover to be applied by the amp, which I think used to default at 80Hz with the Sonos sub so you should definitely get some bass.
Very odd “defect” for the quoted reason; What happens in the app for the Sonos Amp if the RCA wire is plugged only into the Sonos amp, and not connected at the other end to the Sub? A dead end probably, this trouble shooting step, but no harm in trying.
Another experiment - with the wire still not connected to the Sub, if you pull/insert one end of the RCA cable into the Sub out jack of the Amp while bass heavy music is playing, do you hear the bass go missing in the main speakers when the cable is inserted into the Sub out on the Amp? And reappear there after you pull it out?
I have tried just plugging the cable in to the amp (with nothing at the other end), you're right in that it should still trigger the options but alas it doesn't.
So, plugging in the cable - this is where it gets a little strange. The speakers will quieten down and the amp will adjust their crossover - however, a few milliseconds later, they go back to full range. Also happens with the subs connected.
Off the top of my head, even ignoring the lack of sub controls in the app, I would expect the crossover to be applied by the amp, which I think used to default at 80Hz with the Sonos sub so you should definitely get some bass.
Very odd “defect” for the quoted reason; What happens in the app for the Sonos Amp if the RCA wire is plugged only into the Sonos amp, and not connected at the other end to the Sub? A dead end probably, this trouble shooting step, but no harm in trying.
Another experiment - with the wire still not connected to the Sub, if you pull/insert one end of the RCA cable into the Sub out jack of the Amp while bass heavy music is playing, do you hear the bass go missing in the main speakers when the cable is inserted into the Sub out on the Amp? And reappear there after you pull it out?
I have tried just plugging the cable in to the amp (with nothing at the other end), you're right in that it should still trigger the options but alas it doesn't.
So, plugging in the cable - this is where it gets a little strange. The speakers will quieten down and the amp will adjust their crossover - however, a few milliseconds later, they go back to full range. Also happens with the subs connected.
That sounds like the auto detect works, changes some settings, but then for some reason undoes them.
Loose fitting rca connectors are a thing, but I think you’d need to be very unlucky to have multiple cables and none of them work just with the Amps. 
Maybe someone with an Amp and non Sonos sub connected via rca will see the thread and can provide details of how their system is behaving.
I don’t know how many times you’ve called and discussed it with them, but maybe another call to Sonos support with a diagnostic might get a different outcome.
I suggest that you capture this behavior in a diagnostic and report to support again.
Perhaps @Corry P can escalate.
So, plugging in the cable - this is where it gets a little strange. The speakers will quieten down and the amp will adjust their crossover - however, a few milliseconds later, they go back to full range. Also happens with the subs connected.
Interesting and I would be curious to know what the issue was once it gets identified and hopefully resolved. Since all three amps are doing this, it cannot be a hardware defect.
I've got my hands on a newer generation amp and can confirm that the sub outputs work on it, so basically the sub outputs on the S2 "grey/white" amps are broken due to the software update.
