IMO some speaker cables are better than others but there is no correlation between efficacy and price . Having said that . I do not consider internal speaker wire is a fruit-full area in which improvements can be gained . But boy I have tried it on many many occasions and and if my speaker cable was not hard wired direct to the drivers , whose knows I may still be trying it .
Advise on forums vis-à-vis equipment upgrades is paradoxical . Rarely if ever has anybody used the exact same equipment as you in the same room and so the only real valid advise is "try if for yourself" . However , if that was the only advise there would be no forum and we would not benefit from other peoples real experiences .
With this in mind , if I wanted a cheap upgrade I had your system I would sell your speakers and purchase a used pair of vintage infinite baffle speakers with a first or second order x-over of ebay , from the likes of b&O, JPW , AR , Wharfedale or celestion made in the 60s and 70s .