This is my setup and how it works:
AVR: DENON
5.1 and Quad tracks always send the 'rear' info to the "Side Surrounds" speakers. At 90Âș-100Âș
The sound content on the Side Surrounds may be perceived some times as a kind of 'binaural' effect. SO I think, these side surround speakers should be more important, similar characteristics to the fronts, than the Rear/Surround Backs speakers of the 7.1 (at 130Âș or more)
On the other hand, some mixes have enough discrete content for the Surround Backs (130Âș-150Âș), not eclipsed by the side surrounds content, the effect is significantly noticeable if they are located good at the rear.
If I want to listen to Quad, usig the Rears (130Âș-150Âș) to get a more 'square' layout and avoid the intrusive binaural of the sides surrounds, I have to reconfigure the AVR to send the rear channels of the Quad to the rear speakers instead of the 'default' side surround speakers. I can do it because my Denon has Custom Speaker assignemnt. If not, it should be done via Speaker switchers or the like.
I would recommend:
- Use the old speakers to 'test' how you like for different albums/mixes. Even alternate them from Sides to Rears to compare particular mixes, if you are willing to do it.
- Once the 'test' phase has ended (days? Weeks?) Make a Final decision: Locate the Old Speakers where you prefer or ... better... Buy new Speakers of same characteristics of the rest, if you concluded you have a benefit.
This is what I did for the additional 'Wides' to go to 9.1.4 and after testing with old speakers, and exploring what content some mixes send to the Wides, I bought a couple af additional speakers, same brand/model as the rest.