For digital cables it is hard to imagine a cable can make a difference as long as it can pass the signal. Hence the lack of a "boutique" cable market for computer or telecommunications which are massive industries.
For analogue cables, objective data is easy to measure. My view is that a cable can be used to "color" the sound which can make the music more pleasant and enjoyable to an individuals personal tastes. The economical way to go about this is to audition the cable in your system first.
Power cables are a tough one. You don't want to know what sort of conductor is used between the station and your house (kilometers of aluminum and steel). My philosophy is use a power cable that can carry the current (minimal voltage drop) and is shielded so not to pollute any nearby analogue cables with unwanted noise.
As for power conditioners it depends on what you are powering and by a power conditioner, I mean something that will give you a constant AC voltage. For equipment with regulated power supplies there should not be much if any benefit. But if the PS of the equipment is not regulated the internal voltages (think bias) will change proportionately with fluctuations in your mains voltage which changes depending on load on the system.
Just my 2 cents.