RX-DP20VBK/RX-DP15 -- THX Surround EX THX Surround EX is a format that has additional left and right Surround Back channels on the basis of Dolby Digital 5.1 channels. It enhances the sense of depth, provides the sense of smooth movement, and improves the definition of movement of sounds in rear channels -- all to make you feel you are there. The RX-DP20VBK and RX-DP15 come complete with a 7-channel amplifier, making it unnecessary to add on more amps.