Why not just try it in all three locations, and see which you prefer? If you can, I’d recommend a TruePlay tuning of it in each location, but it isn’t required. I moved mine around to several locations, until I found a space that worked best for my personal situation. 

Thanks @Airgetlam

In your opinion then, given the size of the room, you would recommend getting the sub and the beam, rather than just the beam?

My wife hates it when I get on these forums as it always costs me money!! 

‘A’ would be my first choice, followed by 'C'.  Some folk tend to not like a sub located behind them, but that’s maybe a personal thing. A sub low frequency output is said to be non-directional, so it may not matter too much. The real answer is to try all three locations in the room and see which you and (more importantly) your wife prefer.

Heh, that I can understand. Yes, I do think the Beam will sound better with a SUB. But the joy/benefit of a Sonos system is you don’t have to purchase them at the same time, you can add the SUB later, when there is less ‘resistance’ from the SO. I did so, running my PLAYBAR for almost a year before adding a SUB. I was ecstatic about the sound of my PLAYBAR, and was blown away by the addition of the SUB. Much like the positioning issue, it’s really up to you if there is value to spending the money this second, and getting both, or getting the Beam now, and then picking up the SUB in the future. It’s really up to your wallet, and your home situation. You’ll be happy either way.

Thanks both @Airgetlam. @Ken_Griffiths 

As it appears there are some lip syncing issues for some with Samsung TVs I may just get the Beam and see how that goes first. As you say, I can add the sub later if needs be.

I can probably guess my wife would probably prefer B or C anyway as it would be more out of sight, although in position C any rumblings are likely to upset our rather sound sensitive collie, as behind my armchair is his preferred napping spot!

You need to definitely look at this forum regarding lip synch lagging.  I have the Samsung Q80 (2019) and I found a solution that you should look at so you’re not surprised by the problem.  See my other postings.

Is either the sofa at the top of the pic or the other a preferred sitting when watching TV.  You could still place two One’s as surround on either side of that one.   Or put the two surrounds at positions B and C.

Let us know how this progresses for you.  I have the Beam and two One’s for surround.  Don’t have the Sub yet.  Maybe I’ll give it later this year - after Thanksgiving.

Many of the reviews for the Playbase that I own said a sub wasn’t necessary. I enjoyed it as a 5.0 system until Sonos had an offer on the sub and thought I’d take advantage of the trial period by buying direct. Once tuned with Trueplay it was clearly never going back. I’ve owned 5.1 systems before with Kef and Rel subs, both of which could make “thumping “ base louder but none integrated anywhere near as well however I tried to tune them. I’d highly recommend trialling one!