I'm interested in this topic as well. I currently have BL6000s as my rears, BL1s as surrounds, and BL9s are fronts. After installing the Theatre, I might use the Theatre for the fronts as well as center channel. This would free up my 9s to be used as wides or possibly rears -- or even surrounds. I also have a pair of 8000s and 3s and 4000s that I could move into the room. A discussion of how well various beolabs are suited as rear speakers would be great -- especially in light of the new bass management system. BL3s have the advantage of not being obstructed by a sofa. However, they lack bass. BL9s would have the lower drivers blocked by a sofa, but they would add bass to the rear of the room. BL1s were once recommended for rears due to their height. However, now that we can add Atmos height speakers, the height of the BL1s might actually be detrimental to the placement of speakers and get confused with the height channels.

I have a pair of 1s I was considering using also. But, having read that with Atmos a lower level is preferred, as you mentioned, I have decided against using the 1s. My Theatre is due next week so am looking forward to trying the 9s. My dealer is holding a pair of 18s for me that I bought, but if I decide to go with the 9s, will get a refund.

isn't this going to be down just as much to looks / ease of placement than it is by sound? The rears, even with the Theatre, are only going to be doing so much work a lot of the time. One could very easily argue that both 18s and 9s as rears are already well into overkill territory as it is. If it were me, I'd be much more driven by looks / ease of placement considerations. I'll be using 17s as rears with my Theatre (and Bl50s fronts) setup.

how big is your room and how far would the speakers be from your couch?

Room is about 15 feet by 17 feet.  Seating is one end of room. Speakers just to the side  and slightly behind the seating.

Why not just try it out! Every room is different. MM

That is what I am going to do with the 9s once the Theatre arrives. But I have never had an opportunity to hear 18s, but in some ways they might look better in the room.

I'm curious about how the placement of speakers with substantial bass -- such as the Beolab 9s -- work with the new bass management system of the Theatre. This really pulls a lot of Geoff Martin's articles together. I once had two BL2 subwoofers -- one in front and one in the rear. This made quite a big difference. You couldn't really notice WHERE the bass was coming from, but you could sense more balance...and perhaps FEEL it. Literally. However, my neighbors weren't quite fond of my using two BL2 subwoofers so I removed one of them. Yet, there are times that I'm tempted to plug that second bad boy back in, put in the Top Gun UHD, and take The.Highway.To.The. Danger Zone!