Clean up your power supply.
I discovered this last, but it should have been first. Get an electrician to run a dedicated cable from your switchboard to your power outlet for your analogue components (amp, power amp),and another for your digital (CD etc). The induction noise induced by fridges etc is amplified and creates background 'noise'. The changes for me included a bigger soundstage, more bass (even at low volumes), more black (quiet behind the music), and a general ease in the delivery. In terms of bang-for-bucks, this was the biggest improvement over new Transparent interconnects and Purist Audio Proteus speaker cables (the latter gave the best improvement but at a RRP of USD8000, they should).
Power Cords
Definitely work, it's just a question of which one, and how much improvement you get for the $. They often take 100s of hrs to burn-in, so see if you can borrow a few and listen to their effect. I have found the greatest improvement on my amp and pre-amp, rather than CD.
Power Conditioners
You can spend heaps on these, and whilst the good are exceptional, the others are VERY ordinary. (If you can't here the difference it is not for you, regardless of the hype.)
Bang-for-bucks: If you have access to a computer UPS uninteruptible power supply, give that a try to clean up the noise. The best are those which convert your power to DC and then back to AC. If you are drawing heaps of power, you will need one of suitable size, but I would recommend one of 1000kVA or better for amps or pre-amps. The small ones do a great job on CD players.
PS Rememeber it is about listening to your music, not your system.
Cheers