Where to use a pair of interconnects?

Philjolet hit it on the head. Its your ears - expectations, with your system in your home.

PREAMP TO AMP !!!

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.