I would imagine it’s because there’s only one subwoofer signal in a 5.1 system regardless of how many subwoofers you connect, so any digital crossover and/or phase adjustment would apply to both.

For music you could Group the two Amps but that is a lot of expense for what is likely to be a small, if any, improvement in the crossover setings.

Phase makes more sense as that could have an impact. As was said above though there is only one .1 signal so it isn't possible now.

Maybe in a future release Sonos could move the phase control from the primary speaker and make selecting it a function of each Sub.

First off, this question is regarding passive speakers powered buy Sonos AMP, for music listening only, and has nothing to do with Home Theatre. So not really a home Theatre topic. Sorry if I was not clear.

Second: the new Sub is called Sub 4, not Sub Gen 4. Not sure why my description was modified to reflect incorrect terminology.

UPDATE: I went ahead and added a second Sonos AMP, and am now able to control my second sub, the Sub 4, separately from the Sub Gen 3 paired to my first Sonos AMP.  I am experimenting with phase and crossover settings.

ALSO, I am now powering my second set of L/R passive speakers on the second Sonos AMP, (all one room) rather than in parallel on the first Sonos AMP.  A lot more flexibility to adjust EQ and volume now with the second Sonos AMP in the mix. This flexibility is really helpful, since I was limited running two sets of passives off the one AMP.

Comments welcome.  I’ll update again later as I gain more knowledge with the two amp set up.

Hi ​@Ricoh 

I altered the original text of your post in case someone got confused and thought you were asking about a Sonos One Gen 3 (which does not exist) due to the capitalisation of “One”, and changed the Sub 4 to Gen 4 while I was at it merely to make the text more consistent. You are correct in that the current model of Sub is known as Sub 4 rather than Sub Gen 4, but that had honestly escaped my notice until now. 

In addition, Dual Subs is a Home Theatre option and is not available outside of one, so although you use it only for music, because your question is about a Home Theatre feature, this thread is being categorised as Home Theatre.

Apologies for any confusion.

Dual subs certainly is an option outside of home theater. On my amp that is powering a set of left and right passive speakers for music I connected a second sub through the app.

Hi ​@Ricoh 

You have a Home Theatre solution because you are using Amp. Dual Subs are not available with normal, non-HT speakers.

I know we're going round and round here on symantics but......tell me what about my SVS bookshelf speakers, and/or my energy tower speakers, that make them HT speakers, and not standard two channel speakers?

One of amazing benefits of the Sonos amp is the ability to power left and right traditional passive speakers for music listening. My Sonos amp has never sent a single wattage of power for anything home theater.

I think you missed the point.  In your OP, you mentioned connecting “dual Subs” and also mentioned adding a “second Amp”.  So we assumed you were running “dual Subs” through your first Amp, which is only possible when the Amp is used in a Home Theater mode, i.e. connected to a TV via HDMI.  An Amp which isn’t connected via HDMI can only be connected to a single Sub.

Hi ​@Ricoh 

Ok, I think we might be misunderstanding each other. You are right - if you have two speakers attached to your Amp, and no TV, then it is not a Home Theatre setup.

I was saying that this specific thread is in the Home Theatre category because you are discussing Dual Subs, and Dual Subs can only be assigned to a Sonos room if that room’s main device is a HT Primary classed device. These consist of our sound-bars, and Amp. So, Dual Subs is a HT feature, and the only reason it is available to you is because Amp is a HT-Primary class device, regardless of how you use it. If, say, you were using a stereo pair of Fives, or the Amp’s predecessor, Connect:Amp, Dual Subs would not be an option, and this thread would not exist. But, this is purely about where this thread is located and nothing else - it bears no relevance to any settings or procedures available to you. Though, it does appear that my moving this thread into Home Theatre might have coloured some of the replies from the other users above.

I didn’t in fact think it was possible to add Dual Subs to Amp unless there was an active HDMI connection - I’m going to look into this. Please be aware that you may not be able to do this in the future.

Anyway, it sounds like this is all academic now - I hope a little more adjustment will get you to where you want to be with the sound.

I hope this helps.

Actually, I do have a TV connected to the HDMI. Reason being, on occasion I want to listen to a sporting event or a televised concert on my two channel system.

I don't have any surrounds connected, so definitely not a home theater scenario, but it sounds like we have finally arrived on the same page. Thanks a lot for the feedback and information.

Actually, I will double check tonight if I my AMP is connected via HDMI EARC to my TV. 

I may have ditched that connection and instead just used the grouping feature to achieve the task of TV sound over the AMP 2 channel set up.

Okay so I checked the back of my amp, and it does not have an HDMI connected, and it was more than happy to allow two subs, a Gen 3 and a Sub 4. So I understand the amp might be a home theater product, but it does not require, at least at the moment an HDMI connection.