Which IC is more important?

Both the interconnect between source and pre and the one between pre and power are equally important. But each serves a different purpose and the selection of each should reflect its different purpose. The one between pre and power should be as neutral as possible, have great dynamics, great bass and treble extension. In contrast, the one sitting between CD, for example, and pre can be less dynamic and can even exhibit some bass or treble roll-off. It should provide a very clean sound which can even be slightly lean, bright or balanced to give bloom to the upper mids or highs without negatively affecting the overall sound of the system. To demonstrate this, you can use Kimber PBJ, an excellent budget interconnect between CD and Pre. Trying it between pre and power would be tantamount to a disaster in my opinion. There would be no bass and lacking in dynamics. While I believe that there are interconnects that sound better in my system, I hesitate in saying that there are great interconnects and bad interconnects. I use them to fill in what the rest of the system lacks in synergy. Hopefully, for any problem in matching components, there is an interconnect (or a speaker cable) which will fill in what is missing, or cut what there is too much of. Therefore, there is no cable which fits all situations and doubling up on the same cable model or even manufacturer (different models) is almost always a bad idea. You will usually get too much of one thing and the system will tend to sound unbalanced. The only time I have noted where two interconnects from the same manufacturer complemented each other quite well was Acoustic Zen Silver Reference on CD to pre while running Acoustic Zen Matrix Reference on pre to power. But then, all interconnects are somewhat colored, none ever sound perfect, and each only has application to complement the existiing sound of a particular system rather than universally enhancing and matching every system.