You would be able to connect from the S1 Pro's 'LINE OUT' to the Sub1 bass module, if you're wanting to play over Bluetooth. While the S1 Pro does not have an optional HPF, you shouldn't really notice too much of an overlap with this setup.
Yes, that's what I did. Sure it works, but I am picking up some muddy sounds in the duplicated low frequencies when S1 Pro is source "line out" in to the Sub1. When I do the recommended Sub1 line out to S1 Pro, ( or to another top box ), the Sub1 Pro High Pass Filter gets rid of the low frequencies and the top box is much cleaner in sound, especially for an acoustic double bass.
I was really hoping that BOSE could firmware emulate the same in reverse, so that the S1 Pro would self High Pass Filter and let the Sub1 take a clean load below 180Hz. It's on my dream wish list... for the S1 Pro to become a defacto mixer and bluetooth source for the Sub1, while respecting the benefits of a HPF.
I have noticed a lot of ppl in You Tube using the S1 pro as the source for the Sub1, even the local PA shop had it set up like this because it is easier to do !
But here is the real reason.....many small gig players start with an S1 Pro for its great all-round performance, it is an entry level product to the BOSE experience. Then we soon discover that as you go from 50 to 100 ppl, more "oompf " is need, but we don't want to throw away our investment in the S1 Pro, so add the Sub1. By the time we need a dedicated mixer as the Sub1 source, then we are not using S1 Pro anymore - time for the next level upgrade.
Your BOSE marketing ppl would love this, it would be the perfect upgrade feature pathway that your existing S1 Pro users would value. It would bring your customers back again and again for incremental improvements, with each step integrated and matched. BOSE would be the first.
Yes, please pass on to firmware/app engineers - hopefully it is an app update for S1 Pro in "Sub Mode" HPF connection. But this would need to auto reset back to full mode after switching off, just so the S1 Pro does not stay in HPF mode.