NW1: According to the spec section of the manual for your Tascam, the intended assignment for the combo jacks is XLR for microphone input use (which you'd typically rather not want for line-level devices due to rather lowish input impedance) and T(R)S for either line-level or instrument input use (depending on the switch setting). That's a pretty usual assigment, although not all prosumer USB sound interfaces share that. Well, and then you've got the choice: Most commercially availabe RCA adaptors and adaptors cables merely sport a TS plug, so that the outer RCA contact is always connected to both the ring and the sleeve contact. However, some commercially availabe RCA adaptor cable variants use the alternative and sometimes preferable connection variant with a TRS plug in combination with a cable with two inner conductors plus shield, connecting the inner RCA contact to TRS tip, the outer RCA contact to TRS ring and the cable shield to TRS sleeve only, which in some cases can help to prevent ground loops (and of course you could alternatively also make such a cable yourself, if you'd like to try that...).
Greetings from Munich!
Manfred / lini