Greetings and welcome,
You are correct that an LOC is not needed with this powered subwoofer.
Rockford Fosgate® - RFTECH Support
However, the subwoofer does not have any high level outputs to be sent to the speakers. Ideally, you want to tap the front speakers in order to bring the bass sound forward. You have limited capabilities with this product. That is why many who upgrade the 6 speaker system go for a DSP for tuning and a 5 channel amp to get clean power. Thus the expense and that is what you are not looking for now.
To get the output signal going back to the stock speakers, you need to make a parallel connection from the speaker wires coming out of the HU. When you do this, their is a very good chance that will be a lose of signal power. Therefore, it would be better in this case to tap the rears. The rears are full range in the stock 6t speaker system. Whereas, the fronts uses a crossover network between the 6x9 and tweeter. If raise the volume to compensate if you did the front connection, you will start to have distortion. That is why you a better off taping the rears and the RF sub you can always adjust the volume as well.
Hope this helps.
Best for now.
Laredo X, Trail Rated, QTii, Sele-Terrian, Quadra Lift, Off-Road Adventure Package ii, Nokian Rotiva AT, Tow, AFE CAI, Custom Sound System ( HAT speakers, JLAudio , Pioneer , JBL MS-8 & RF 3.sixty.3 -- see the writeup link below for more details)