Where to use a pair of interconnects?

No two systems or listeners are the same.

Let your ears decide where the ICs provide the best performance boost in
your system.

In other words, go with Philjolet's recommendation.

Here's the logic:

If you only have one source, it seems that the best should be on the source. Once a signal is altered, it can't be fixed because the original information is changed and the next piece of equipment can't guess what it should be.

If you have more than one source, it seems like it should be on the preamp to amp so that all of your sources receive it's benefit.

Now here's the real world:

You need to go with Philjolet and just try them. It's your system, your ears and you know what you want or like.

I agree, where it sounds the best.
However in my three different systems, the interconnects sound the best between my preamp and amp.

I reject the question. I just went ahead and bought the other ICs I needed. ;-)

Seriously, though. I have been through this before to know that it doesn't really make that much difference if you are only upgrading one IC. If you let this drive you nuts, you will go through the same exercise each time you add another set of ICs. And it will be different each time. You really won't hear the full benefits until you get them all upgraded anyway.

I prefer my best IC going from sources (CD, DVD) to my pre/pro.