Speaker cables to make a huge difference. For me, this improvement was more than interconnects and PC's. My first expensive speaker cable was an old AudioQuest Volcano. It brought out the missing bass with my then Revel Salon 1. I even chopped it up in half (done by AQ) and used with mono blocks, even better sound (mostly the monos). Surprisingly, after not using it for 5 years and forgetting about them, I tried TMRAudio to sell them and got an incredible price for them.
I currently use Audience FrontRow and Conductor SE speaker cables and sometimes the Benchmark speaker cables, if my amps have SpeakON terminations. I need to buy another set of speaker cables in the future and it will be FrontRow once more.