Selecting Rear Surrounds for 5.1 system

Atmos sends full bandwidth to all speaker channels. So there is justification in using good speakers on all channels. If you highpass to a sub you only need to get down to the crossover point (normally 80hz). 
 

it is just a matter of how much you care. Rear speaker quality does matter, I find I just don’t care if surround is accurate. I just want it not to break up and be pretty flat   

Honestly I would looked for a used pair that more or less matches your fronts. Keep it cheap and replace that Polk sub. A good sub matters a lot more. 

@sls883 

I have not bought the B&W for rear.  I just went to Best Buy and spoke with the Magnolia room person and got a demo.

I've got JM Labs, now Focal, Electra's. They do a great job. 

I went over the Best Buy meantime and asked the Magnolia room person the same. He said it’s best to balance the room with B&W 704 or 705. He was against using KEFs or Polks or Martin Logans in the back when 702s were in front. He said that they perform quite differently. They sell KEFs and MLs also.

Of course he’s gonna say that because he wants to sell you more speakers and that’s the easy layup answer. I worked at Magnolia briefly and have been to several others, and they are mostly just hacks who have very little personal or real experience in high-end audio. When I cobbled together my HT system I used my wife’s Polk monitors from the 90s that were not in any way matched to my fronts and they worked perfectly fine for creating a very immersive movie experience. If you wanna blow a big wad on the rears have at it, but my opinion and experience is that it is not necessary. Save the money there and put the rest toward getting a better (or second) sub or center speaker where it’ll make a much bigger difference.

i sorta agree that unless you're listening to multichannel music the rears are less critical, but all things being equal i'd stay with b&w for them--their 707 models aren't too expensive and should work well.