The following is my experience (limited) with cables:
Interconnects:
I replaced my out of the box interconnects on an earlier system for a Chord Crimson. The sound became a lot more detailed, however it did seem a little lean in the Bass department.
This was then changed for a Chord Chameleon Silver screen which addressed the lack of bass issue.
So IMO interconnects do make a difference and you can tweak the overall sound to suit your tastes, but only within the limitations of your equipment.
Speaker Cable:
I swapped old cheapy wire for some around £5 per meter years ago and the difference was pretty negligable. However I then upgraded to Chord Silverscreen, which made an incredible difference to all sounds, almost like the cotton wool had been removed of the microphones used when recording.
My last upgrade was to Naim NAC A5, again it is a brilliant cable but I cannot comment subjectively on how it has affected my current system as I have never heard it with anything else.
Mains Products:
I changed my standard extension lead for my AV gear to a Tacima Conditioner. The differences again were not subtle. PQ improved a lot less noisy. The sound from my reciever was clearer and more defined.....however I felt the tacima was restricting the dynamics of my Reciever so I plugged that back into the wall socket.
The upgrade that has surprised me the most has been my most recent which was a Chord PowerChord fitted to my CDP. The sounstaging iproved as did the dynamics and the detail. I dont know how, baffles me but it did make a HUGE difference.
Fitted another Chord PowerChord to my TV and the picture quality has improved very slightly, however the sound from the TV speakers is now a lot clearer.
So as you can see I think cables do make a huge difference, but as others have said you are limited by the quality of your electronics, the quality of your hearing / sight, and in the case of mains equipment the quality of the incoming mains and the wiring of your house.
I am adding a seperate radial circuit for my HIFI to seperate from the normal ring main next month so I will feed back on that one.