I won't enter an opinion for the idea of power cables/conditioners. Instead, as probably has been stated, do all that you can physically with the listening environment first. I should take the advice that i give here. When reading such books as Jim Smiths 'Get better sound' there are many ideas that can be tried out for free, and if they work as prescribed you are ahead of the game before spending extra money on other attempts to improve your systems performance at large.
Tweaks are fine, if you so desire, after the physics that can be dealt with are dealt with first.