The Flex balanced is balanced in/out. If you'd like to use an unbalanced amplifier (e.g. Powered subwoofer) downstream of the Flex series, here is the recommended TRS to RCA cable to minimize detrimental effect on the audio quality. 

Note how the TRS side is only connected with Tip and Sleeve while RCA side is not using the shield.

WARNING: This adaptor(TRS to RCA) is ONLY to be used for the Flex OUTPUTS. We've noticed a performance drop (15dB SINAD drop) when using such adaptor for INPUTS of the FLEX. For that reason, we do not recommend customers to use an unbalanced to balanced cable on the "input" side of the Flex. If you want to have unbalanced in/out for both inputs and outputs, it's strongly recommended that you just use the miniDSP Flex unbalanced version. The miniDSP Flex was engineered with best performance in mind and it's therefore our intention to stay consistent on making sure best audio quality is kept for this platform! 

Once again to clarify, there isn't any performance drop for OUTPUTS. i.e. TRS to RCA cable for an unbalanced amplifier or subwoofer if perfectly fine. This limitation is for INPUTS only. i.e. RCA source to TRS input of the Flex.