ARC ULTRA bass can be more aggressive than ARC bass. Some users have remarked that MINI doesn’t add much to ULTRA. If you add 300’s, you’ll have full ATMOS. A SUB would just augment the bass. Of course if you are an absolute bass freak and watch a lot of thundering action films … .

Thanks, bass does seem ok to me on the Ultra. Wasn't sure what the mini would add. I have heard it helps the Ultra to perform better as it takes that low end away from it...

Yes, that’s always the case. When adding a subwoofer, the midrange is clearer. Also in the mix is your room configuration. If you don’t have an optimum spot for the 300’s, ATMOS will suffer and you may enjoy bass extension and midrange improvement more than a half-baked attempt at ATMOS. Also, what percentage of your movie watching includes ATMOS films?

My couch is to right of the TV, it runs parallel with it, so my thinking was to put two speakers behind that couch on stands. There's enough space as there's a window behind me. Hoping that gives me the immersive experience I'm looking for, after trueplay

Off axis front’s would be a big downer for me and I don’t like surrounds to be very close to me. You could argue that the 300’s will help a little because they can throw some sound out the top, but only for ATMOS tracks.

Not an ideal set up I know! Just hoping it will give me something extra. They will be a couple of feet back from me so here's hoping it works. 

My surrounds are poorly located but still sound pretty good. 

I'd worry more about the off axis with the front than surrounds, but you can only do so much in difficult rooms.  Maybe a  combo TV Arc pivot mount that you can pull out to get things closer to optimum when listening?

Technically they will still be behind me but I know what you mean as one will be a lot closer as I turn to my right to watch the TV. The other one would then be behind me to the left. I could move the couch back to where it faces the TV, but it's about 15ft away then. I assume it would be better soundwise,  though.