Upgrade Speaker Cables or Interconnects

I would try powercords, followed by speaker cables, and finally interconnect cables. All systems are different, that's why I say "try", so your ears can determine the best upgrade.

Generally- almost always in fact- the best approach is to upgrade the weakest component. You're thinking cables. So which one is worst? Since you neglect to even mention power cords then I assume you have neglected to upgrade and are using the freebie crap that came in the box. Am I warm?

BTW once you get good power cords on everything then look at fuses, cones, cable elevators, component rack or shelf, as well as other highly effective tweaks like ECT and HFT.

Update to my post: I am using MIT Z-cord power cables, amp and cd player, shunyata venom PS8 with venom HC power cord.

 More will say speaker cable makes bigger difference than IC. 
Some will say the opposite