Whats more important, source cables or amp cables?

I'd like to save some time demo'ing my own system for hours and get some experts to tell me what to do. What's most important, the cable between the source and the pre-pro, or the pre-pro and the amp(s). Does it make more difference in the sound to place my better pair of XLRs before or after the pre. I have a small Krell system, DVD Std, HTS 7.1, and 2250/3250 KAVs. I asked Krell and he told me they are both important, but that didn't really help since finding another pair of Crecendos is easier said than done. I have one pair, and 2 pair of Serenades, which don't suck, but from what I've read, aren't near as good as the Crecendos.

Any help or explanations would be beneficial at this point. Thanks AGers