6.1 sound tracks were 1st introduced by THX...
Next Dolby made it 6.1 EX but shortly thereafter came out
against it advising the center back channel collapsed the separation of the rear channels. Next came the AVRs with 7.1 channels, and under the Dolby license, designed the EX sound tracks to be processed as Dolby Digital EX 7.1 preserving the channel separation..
Quote from the Dolby license manual..
"While Dolby Digital EX decodes 5.1-channel programs into a 6.1-channel format, it is common for products to offer accommodation for both 6.1- and 7.1-channel speaker configurations. In the case of a 7.1-channel system, the Bs signal is fed equally to two separate speakers (LB and RB). To account for the resulting increase in loudness, the signal splitter applies 3 dB of attenuation to the LB and RB signals. The use of two back speakers is recommended rather than one, as this helps spread the effect more evenly in the listening room..."
Just my $0.02...