What was the first power cable that you noticed a difference in the sound?

Power cables don’t really make a huge difference with an amp. Components like Streamers, is where p/c upgrade is noticeable.

nordost  Heimdall works  for  me 

The only really noticeable change for me was, an Isotech power cord to my Modwright power supply.

Shunyata Venom-X EF

With patience these can be found used for $500

@bigtwin Yes, that is my thought.   I have a friend who rebuilt his tube amp to sits on top of a bank of capacitors twice the size of the amp.  All the power cable has to do is keep the capacitors charged up. this is a very flat power demand.  

Cable deniers usually base their claims on their superficial understanding of electricity.  If you amp's average wattage is 240 watts, then you only need 2 amps and an 18 gauge cable is plenty big so 16 awg is "oversized".  What they miss is the extreme dynamics of the load in an amp where 50 amps might be needed for a picosecond.  

So if you have power available downstream of the power cable that can respond to these dynamic demands,  then the amp will be insensitive to power cables.  The cable only has to provide the nameplate wattage of the amp, which is relatively low. 

OTOH, I have a tiny little tube amp, a Sophia Baby, that weights 13 lbs.  I thought surely a 14 awg cable would be plenty for it.  after a couple of days of disappointing bass a friend suggested a bigger cable and immediate improvement.  In retrospect, it makes sense since this small amp has a minimal power supply.

Notice that I have ignored all the magic dust in $4000 cables.  All I have addressed is cable size.

Jerry