what's the most vital cable in a system?

I would normally say the most important cable in my experience is the Pre-amp to Power Amp Link, but you have an integrated amp. Second for me and first for you in my opinion is the link between the MSB DAC and you amplifier. Then the speaker cables.

There's an old audiophile saying; "The best a system can sound is related to its weakest link". With this in mind, there may not be a valid answer to your question. Try not to skimp on any one given connection, this will probably give you the most bang for the buck. A final point, trust YOUR OWN ears, when you make those cable comparisons. There are quite a few "recommended" cables, that are real stinkers (especially, if you factor in their cost). I have making my own cables (based on a design involving 18 audiophiles and 3 electronic designers), which sound better than cables 10x their cost, i.e. IT DOES NOT TAKE HUNDREDS AND HUNDREDS AND HUNDREDS OF DOLLARS, TO PRODUCE AN EXCELLENT SOUNDING CABLE!!

I agree with Ehider that the system is only as good as the weakest link. However, with all the components in a system being comparable, I would say that the cables closest to the source is the most important, then from preamp to amp, then the speaker cables. If you think about it, if you use crappy cables between the source and the preamp, no matter how wonderful the cable you use between the preamp and the amp, the sound is already degraded or colored. You will just be producing accurate sounds of what your cheapy cables produced. Work from the source-preamp to preamp-amp to speaker cables.

My understanding is that the lower the voltage a cable carries (line level) the more prone it is to EMF/RFI. This seems logical because microvolts can be more easily disturbed than 'macrovolts', so it follows that good interconnects are 'more important' than speaker cables. That said, I think the question is a conundrum and somewhat like asking which wheel is the most important on a motorcycle. Both front and back have an important function, and regardless of how good one is, without the other, it's virtually worthless. The efficiency of any system or process is determined by the weakest function within that process (as stated by Ehider, above), and you would be well served to follow his(or her) advice on trusting your ears above all else. Good luck!

Kweisner, if lower line voltage cables are prone to more EMF/RFI, how seriosely are the conections fron power supply to unit affected? Any thoughts on how to improve or upgrade? P.S. I also believe you start at the source and work down the chain. BUT DO UPGRADE!!!