what's the most vital cable in a system?

All of the cables are vital 'cuz the system won't work without them :-) Out of curiosity, what is the rationale for the interconnect from preamp to amp being the more important than the interconnect from source component to preamp? When I upgrade interconnects, I do all the line level interconnects in one fell swoop (or is it swell foop? whatever...) I think that if one considers signal levels, driving source power involved, and susceptibibility of the receiving end to cable-induced signal degradations, the hierarchy of cables having most effect to least are: (1) low level analog interconnects such as from turntable to phono amp (2a) line level analog interconnects such as from CD player to preamp, DAC to preamp (2b) line level analog interconnects from preamp to amp, (3) speaker cables, and finally, at a very long distance behind the others, (4) AC power cables and (5) digital cables, the latter especially for having insignificant incremental effect once basic electrical performance specs are met. It also seems that (4) and (5) are subject of much more mystery and heated debate these days than (1), (2), and (3). This observation is consistent with the First Law of Audiophile Discussion: heat of the debate is inversely proportional to size of the effect :-)

I think what your asking is what cable has the most sonic impact...in that order. With that, it would be your speaker cables by far. Than your interconnects. The ones closer to the source & than on down the line in that order of impact. Lastly power cables. I hope I've understood & answered your question.

I would recommend that you upgrade all of your digital connections to true 75 ohm cable and connectors. You should replace all of your digital RCA connectors with 75 ohm BNC connectors and then use DH Labs D-75 cable with BNC connectors. This was recommended by Gary Galo in Audio Electronics. I have done this to several of my systems and the improvement is better than adding a DIP. The only thing that I have found better is to directly solder Illuminati Ohm Aha 75 ohm cable directly to the circuit boards of my DIP and processor. The sound is terrific, loads of ambience with a natural sound.

Gemini, try Monarchy's solid teflon core digital cable with your DIP. If you don't like it you can retrun it with no problem. I'm running it both from my transport to the DIP and DIP to my DAC (BNC outlet with adapter). I was really surprised at how good it is. Something to do with the teflon core. Actually replaced Transparent with it. There are two versions, one more expensive than the other. I went with the less expensive at CC Poon's suggestion. Less than $100 each.

Source to pre amp is most important.