Use RCA Y-splitters, connect the single end out from the pre-outs left and right, feeding one of each of the dual ends into an external amp left and right and the other of the dual ends into the main in of the RR2150 left and right
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Main HT <> 976 <> 7200 <> Ultra-X12 <> sapphire ST2 mkII RF/LF <> sapphire SC center
sapphire ST2 mkII SBR/SBL <> sapphire SS SR/SL <> Verizon/DVR <> Vizio M70Q7 <> Oppo UDP-203 <> Amazon 4k Fire TV