I’m concerned that the efficiencies of the subwoofer and speakers don’t match well enough to be driven by the same amplifier without some means of adjusting levels. On the other hand, the subwoffer is more efficient and if you like bass, this could be a plus.

Minor disconnect between the original poster’s thought process and what @buzz wrote:  @stas , there is virtually *no* power being output through the Sonos Amp’s (separate) subwoofer RCA jack.  It’s a line-level output intended for a powered sub, but the N.E.A.R. IGS’s are passive.  As @buzz says, you would have to connect both the sub and the main speakers to Sonos Amp’s speaker output terminals, and it’s not clear that would work well.  This is especially true because N.E.A.R. actually recommends a Low Pass Filter, despite the fact that their speaker itself passively limits somewhat the frequencies it will play -- and it’s mono.  (Also, we presume you meant an IGS12, unless there’s a new 13-inch sub model?)

New England Audio Resource makes some incredibly robust outdoor speakers, but they aren’t cheap.  If you’re spending that kind of money, why not get the N.E.A.R. 6XL amplifier to drive the sub?  Or you could get something inexpensive like the Rockville unit someone else here mentioned, and it would probably be fine too.  Hope this helps (and I didn’t mistake your meaning).

�Edit: Sorry, I missed what @buzz might have been implying: You could live without the sub, because the N.E.A.R. IG8 does have a respectable bass output all by itself, unlike some of those random “rocks”.]

Sorry, I wrote too fast. Connecting a subwoofer directly to AMP would not be a good idea because you don’t want to send a full range signal to a subwoofer. One would need to insert a “crossover” network between AMP and the speakers in order to steer lows to the subwoofer and the remainder to the satellite speakers.

As tracker suggests, a better plan would be to use a separate amplifier for the subwoofer. AMP supports this with a special subwoofer output to the external amplifier. Once the subwoofer amplifier is connected to AMP, AMP will split the frequencies appropriately for the subwoofer and the satellites.