Power - where to start? Mains, chords, conditioner, filter?

The OP is an apartment dweller - no dedicated lines, only better outlets and power cords. Yeah, conditioners and regenerators is a complicated subject.
In any case, nothing will create a situation of perfect power, big custom batteries might come close. The best place to start is to have equipment with great power supply. Then power cords and outlets.
Personally, I am finished with this stuff for now, I simply need much better amp and motor controller for the turntable.

I heard IsoTek power conditioners at an audio show and they made the sound better. They have solution from a couple of hundred dollar to about $5000. The more you pay the more advanced they are. You will have to try in your home to see if they are worth it for you.

@williewonka the Yeti don't seem to be made for audio. It probably works great anyway. If you want to try a (much) more expensive version of the same idea, made for audio, there is Stromtank.

http://www.enjoythemusic.com/superioraudio/equipment/0218/Stromtank_S2500_Battery_Power_Review.htm

I've had an amazing reduction in noisefloor from my torus avr20. Monster of a unit...but non-subtle changes. Feel free to PM me if youre interested in learning more.

For all the nay sayers of upgrading cables, using power re-generators, and dedicated power circuits, think the process through. ALL AC voltages have noise and harmonics, which gets induced to any electrical component near by. So buying high quality cables which shield the radiation of this noise will help reduce the overall noise floor of the surrounding components. Re-generators take the power and completely remakes it to a pure sine wave  60Hz 120 VAC for the U.S.. Using dedicated power circuits removes all the possible external devices such as fans, transformers, and all the other crap we have plugged into our households, however due to the common neutral ground configuration this is only a minor improvement overall. 

Also, I would great suggest using a power regenerator, taking your house AC and completely "regenerating" and removing all the harmonics and noise in it. 
I have used a PS Audio re-generator, which made incredible improvement "removing" noise, it didn't change the sound, it made the sound cleaner and have more clarity. I also replaced all my power cables with well made high shielding AC cables to block any noise that each component might generate from their power supplies. 

These 2 upgrades made significant improvements in the clarity of my music and enjoyment.