cable connection

Preamp and source for me. Second choice would be from the wall outlet to the amp. 
Let’s see how many different options we get on this😀

Source <> preamp
Once you lose data upstream, you can't retrieve it downstream. 

Not mentioned, but if the system has a record player, the tonearm internal wiring if it needs it. An uninterrupted run from cartridge clips to RCA plugs.

Source to pre-amp/integrated amp.

Thankfully a choice I will never have to make, having been smart enough my whole career to run integrated amps. But if I ever did go separates I would like to think I'd be smart enough to know the best cable goes between the pre and power. That way every source always goes through your best IC. The idea of once its lost upstream is a red herring. At least I think that's what its called. Do herring even swim upstream? Nevermind. The herring is a MacGuffin anyway. 

Power cable wall to amp would be my first choice, then power cables to components, then moving on to interconnects.

I started off spending a lot of effort on interconnects never focusing on power.  When I demoed a full loom of Ansuz power cables and conditioner I was floored at the difference it made in my system.  It completely changed the sound characteristic to the better.

I’m with eziggy on this one. Power first. It can make all the difference. Then pre>amp...

Hi,
i would expect the preamp->power amp to have the bigger impact. Consider them as one unit.

Get Purist preamp/amp ASAP!!!!

Upgrade from what ?

I think the results can be system dependent...but when I used Transparent Audio cables I agreed with their upgrade advice of first source to pre, then speaker cables, then pre to amp...

Assuming interconnects are currently identical, then between CD and preamp (here the Linn principle would apply).

close score: . 6 for pre - source/  4 for pre-amp

I would go pre > amp. Like millercarbon said, then everything goes through your best pipe.

I will buy the cable to experiment, let my ears to decide where to put it.