Consultancy system extension ?!?!?!

Hi people, I want to expand my anöage below a little bit.Not that I would be somehow dissatisfied,no for God’s sake everything is super cool.But vlt a wenir rear fill back would be bad net. So concretely I wanted to install 2 coaxe back so I got back 13er. and then another end level 2 channel. Now come my questions: 1.Should I buy a new end stage for my subi and put the coaxe on the existing one??? 2.Which coaxe for 13er §. If the end stage only for the coaxe then which achos want to spend net more than 300€ is that feasible?? and music ricvhtig is Hip Hop & a little techno Thanks already in advance