If a marketing move would prevent more purchases it would be a really stupid move…..

Not completely sure what you want though. The Sonos Sub is enabled when using the Amp for surround with third party speakers. It is bonded with the Arc, Beam or Amp used for fronts.

I believe he wants the sub port on the amp enabled while the amp is in surround mode, so that he can use a 3rd party sub.

I don’t think you can conclude it’s an easy change, only disabled for marketing reasons though.  As it stands now the Arc/Beam is currently configured with or without a Sonos sub.  If no sub, then it plays the bass frequencies itself.  With sub, it sends bass to the sub and adjust it’s own speakers to no longer play those frequencies.  Now you’re talking about a 3rd scenario where the Arc/Beam needs to be told that the amp for surrounds has a sub attached to it, so it should adjust it’s on playback accordingly and send the L/R surround channels AND sub channel to the amp wirelessly.  Not sure that the amp is built to handle 3 channels of audio like that over wireless, and process it accordingly.  Maybe, but I certainly couldn’t conclude that it’s a trivial matter. 

Note, this is not the same as when a amp w/sub attached is playing music in a group situation.  The amp would receive the 2 stereo channels and extract the bass from that.  It can’t do that when it’s doing surround duty as the bass is a part of the front channels, not rear surround channels.

As noted, this just doesn’t work architecturally. The sub output is low-pass filtered off the signal sent to the Amp’s speaker terminals. In this case the Amp’s input signal is for the surround channels. The master home theatre player doesn’t send the .1/LFE to the surrounds. 

Yep, I shouldn’t have referred to the sub’s output as a ‘channel’ but the principle is similar.

Yeah ok I see what your saying... At this price point to purchase the Amp Aus$ 999.00 it’s quite an expensive device to not have some features that could handle this configuration. It just seems to me that it’s purpose built to work with the Sonos family and not in their scope to work with 3rb party speaker scenarios, alot of us are looking to replace on old 5.1 channel home theatre receiver with the Amp and our old speakers but it doesn't fit that space. When you look at the outputs it looks like it will work until you purchase it and find out it doesn’t

I don’t think there are any amps that can do what you’re asking at any price. 

The earlier versions of the amp were not originally designed for surround sound duty at all, but the ability was added later on.  And while I’m sure Sonos would rather you buy their sub instead of a 3rd party sub, you can use a 3rd party sub when the amp is setup for just music or is directly connected to the TV to play the front audio channels.  This covers the vast majority of cases.  

As far as assuming that every port on the Amp is active under all scenarios, I get why you’d make that assumption, but it just isn’t the case for the Sonos amp and many other amps/receivers out there.  It’s always best to do some research and ask questions if needed.   

The rear Amp isn’t the sole issue. The master HT player (which controls the decode) simply doesn’t send the .1/LFE channel to the rear speakers.

It’s not just a case of tweaking a few lines of code in the rear Amp. It would be a major overhaul of the HT architecture.

If you want to use a 3rd party sub then an Amp needs to be the master player.