Best cable placement

To me it would make sense to put the better of the two cables between your preamp and power amp, this way any source connected to the preamp would benefit from the better cable instead of just one component.

Transparent used to recommend (have not checked in a few years), cable importance/upgrade order is IC from Source to Pre first, then Speaker Cables, then IC from Pre to Amp...

I’ve tried this both ways and found putting the better or more revealing cable closest to the source was much better.  Of course it’s easy for you to just try for yourself and see what you think. 

When I was upgrading I upgraded ICs source to pre then pre to amp.  Did the same with power cables by the way.

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.