If you have not done Trueplay, try doing it. If you have done Advanced Trueplay, retune with just the basic to see if it makes a difference 

I have the exact same issue with my sub 4 and Arc.  I can’t even tell the sub is on unless it’s set at +15 in the app and I’m listening to music with a lot of bass in it. I have tried everything - including exchanging the Arc (Costco) and the sub.  It’s very frustrating and has to be software-related.  It’s like the Arc is handling bass that it should be passing along to the sub.  I have a call with a level 2 Sonos tech scheduled next week to discuss the issue, but I don’t know how this can be fixed.  I have tried everything...

One additional change I made last night was to increase/max the bass in the EQ and max it in the Sub Audio to +15 and I felt the difference somewhat as I was watching a show.  Based on the show’s content there were times I would realize the base difference. Again, underwhelming as a whole.  

The effects of a Sub should be subtle. Not saying there”s nothing wrong with your set ups, but you should not “hear” the Sub.

I guess that was part of my first posting on this - were my expectations unreasonable?   Exactly when should I expect to “hear” a difference.  Otherwise, whats the point of spending $700 on this add-on?

one thought I had is if I added the two Era 300’s, would it become more noticeable?

The thing with subwoofers is that the placement is very important. If you walk around the room, you might find spots where there is a lot of bass while in other spots there’s literally nothing. This is due to a thing called room modes. So I would advise to test different spots, there is even a technique called the subwoofer crawl which might help: https://www.svsound.com/blogs/subwoofer-setup-and-tuning/75365187-the-art-of-subwoofer-placement

Fantastic - thank you!