Where to use a pair of interconnects?

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.

It depends on many different factors.

You just have to try it in different places, compare, and decide.

Currently my most expensive IC (Harmonic Technologies Truth Link)is between my DAC and Pre-amp because that is where it lends itself best with my current set-up. In the past, it has been used elsewhere.

You tell us - source/pre, pre/amp. Which sounds best? Can you even discern a difference?