If you place a 100 near one of the chairs, that 100 will dominate that chair. I think that the best compromise will be to place the 100’s near the sofa, although there is some risk that the left 100 will be bumped by door foot traffic. Consider wall mounting the 100’s.

You could try temporary stands in different positions to see what you like.

Short stands at the ends of the couch would be simple. Wall mounts above that spot, but still at ear level, are possible.

If wall mounting closer to the room's corners might be good but you'll be higher than optimum to clear the doorway.

Try the options and listen, it is your ears that matter.

Thanks for the replies.

I don’t have the speakers yet so can’t test out.  I want to make sure it can work before committing.

If the speakers were placed on stands either side of the sofa, how would the surround effects sound for those sitting in the arm chairs (in other words, would it still ‘work’ for them, or only those on the sofa)?

This might be a very basic question, but I’m quite new to this!

Many thanks

Surround placement is pretty flexible if you just want some surround sound. My surrounds are horribly placed, too close together, too high and pointing the wrong directions. I'm quite satisfied with the sound I'm getting given the limitations of my room. Same for my seating distance from my Arc, too far away. Not changeable without a new house and spouse.

I'd suggest you go for the same standard, sounding as good as possible, to your ears, in your room and, being where you can live with them. Perfect isn't going to happen in your room but pretty decent could.

At the ends of the couch both chairs will be out of the sweet spot for the rear channels, they are likely already out of the sweet spot for your Arc. I'm betting it still sounds pretty good in either of them. Your couch is also likely out of the optimum position, too far away, again though it probably sounds pretty good.

Above the chair and door will widen the rear sweet spot, not enough to include both chairs, but a bit better. It will suffer some by being too high. Again lrt your ears judge.

Try them, try multiple positions and if nothing satisfies you return them.

You likely won't, I have a pair of 300s I couldn't adequately place (spouse was not happy, not a sound issue) as surrounds so they moved to another room as an excellent Atmos/stereo pair and the 100s replaced them.