The answer to this question is that you must come out of your audio source into the sub FIRST. The sub has TRS or RCA so which ever you can get your source converted to will work. Then from your sub outs (TRS or RCA) to the inputs on the back of the powered main speaker. You are correct in that the 5 series speakers are completely unlike the 3.5s in that the 5's have a sub-out.
So, for practical application, if you are using a CD player with RCA outs, or a computer with a 3.5mm out - you can plug the CD player RCA plugs into the inputs on the back of the sub. For the 3.5mm, use a 3.5mm to RCA splitter cable and then you can plug those RCA jacks into the back of the sub. You could also use a DAC of course, but it ends up just the same - the DAC has to be connected to the sub first - unless your DAC has a sub-out. Good luck.