Adding the Sub or Sub Mini will help. It will relieve the Arc of the lower bass frequencies and allow it to concentrate on the mid/upper range. With the Arc, the Sub is usually recommended over the Sub Mini.

Thanks for the quick reply.  I’m concerned that the Sub will be overkill and create rumbling, which I don’t really want in the bedroom.  Also, the Mini’s size is preferable, however if the Mini will not help, then I’ll go with the Sub Gen 3.

Sub Mini with Arc in a smaller room should be fine. 
 

From FAQ’s:

Will Sub Mini pair with Arc?

Yes. Sub Mini works best with Arc in a smaller room and at low-moderate volume levels. If you have a larger space or intend to listen at higher volume levels, Sub will provide a better experience.

Is it possible that when I added the Era300s, some of the lower tones were taken out of the Arc?

With just the Arc and no surround speakers, the side-firing drivers on the Arc play the surround channel audio. But when you add the Era 300s as surrounds, the Era 300s take over the surround channel audio from the Arc. With Era 300s as surrounds, I believe, with 5.1 audio, the Arc’s side-firing drivers now support the front left and front right audio. And with 7.1 and Dolby Atmos audio, the side-firing drivers on the Arc are now used to support the left and right side surround channels of the Era 300s. So you could just be hearing this difference.

Thank you. That explanation helps. It’s all new to me, even tho thought I did my research. 

If I have a Sub Mini paired with my Arc and Era300s, then turn off the Mini, will the Arc revert back to producing the Mini’s bass or will it always act like the Mini is doing its job?

It would go back to playing the bass, as it would ‘see’ that the Sub (mini or not) was gone. 

Thanks. That what I was hoping.