Best cable placement

In my experience, it all depends on the synergy. In most cases in my system(and different variations of it), the best cable made the biggest difference between preamp and amp. But because it can also reveal the shortcomings upstream, experiment and hear for yourself what it does and where it sounds best. 

@soix

+1 I’d use this as a starting point. I would leave it in my system that way for a couple weeks at least. Until you know the sound completely… two months is good as well. Then switch them. To me this kind of comparison is the way to hear subtle differences. 
 

From my source (DAC) into my preamp has made the most significant improvement by far. What is lost from the source can't be made up elsewhere. 

Thanks for all the insight. 

I'm going to try it between the preamp and amplifier first. I would think that that cable would be the most important, but I don't know. It would seem if the better cable was upstream, and the lesser cable downstream, it would seem the lesser cable would negate the better one. 

Would it be true, if the better cable is between the preamp and amp. That all of the components would benefit from it being as it's the last IC cable in the chain?

Whereas, if the best cable was between my Dac and pre amp, that circuit would be the only one to benefit from it. Whereas the other way all components would benefit from it....

I will be trying both ways, and will report back....

It would seem if the better cable was upstream, and the lesser cable downstream, it would seem the lesser cable would negate the better one. 

That was not my experience.  I found that if I put the lesser cable upstream it choked off the better cable more downstream than if I did it the other way around.  But, the great thing is you get to try it both ways and hear for yourself, which is all that matters.