Best answer by Chris
The Connect:Amp is only able to receive a stereo signal (R/L). In this case the R/L is the right and left surround sound. There is no dedicated chanel to send to the Subwoofer.
The original design of the Connect:Amp was not to be used in the manner of surrounds. Its original hardware takes a stereo signal. Filters it through a crossover - sending higher frequencies to left/right channel speakers and the lower range to the subwoofer output. So the subwoofer output sits between the stereo input and the speaker output (just as a normal sub would). There is no direct channel to the subwoofer just a processing of the stereo signal.
Therefore, there is no reason to have the subwoofer active. There is no reason to filter out the low end output from the surround sound mix (as the surround sound mix doesn't have the LFE channel or really any low end to speak of).
It is an internal hardware issue that makes it impossible for the Connect:Amp to pass a discrete LFE channel in the form of the .1 signal from a surround sound setup. It just physically is not built to do that - so no asking for it will solve the issue.
A new Connect:Amp model designed to pass a discrete signal would be required.
Hope this makes sense as Sonos has no way to add this ability to the current model.