Power Conditioner vs A/C cables

Everything depends on money and your priorities.  I have always used after market power cables on everything.  They are definitely not my 1st priority.  After components, IC's make significant difference, followed by good isolation tweaks to help clean up everything, at that point, I begin auditioning different power cables and their various grades before purchasing.  They do make a difference and influence system quality.

“Is it necessary to buy expensive power cables downstream of a high-end power conditioner?”

Neccessary - no

Preferred - maybe 

It's a matter of personal preferences and ability.
Some people for whatever reason don't hear a difference while using after-market power cords, others do hear a difference. Just depends on which side of the fence you're on?

@phantom_av,

I purchased an Elgar power conditioner many years from a military surplus sale for about 40 dollars. I used it with Carver and Soundcraftsman amps. 
It was heavy and at the time we were moving, so I gave it to my best friend. I don’t recall the model. I don’t think I appreciated the sound enhancement it provided during the time I owned it. 

I'm with ghdprentice. If all critical listening could be done late Sunday nights, regeneration and fancy power cords would hardly be an issue.