Which cable is most important to spend money on...

The basic idea here is to first upgrade the cable(s) that will reveal the true sonic change(s) due to subsequent upgrades (if that makes sense ;--)

So with that in mind, I'd first put a heavy duty (10 AWG wire gauge) shielded power cord on the amp and (if you have one) the power conditioner/regenerator.

Next a good quality pair of ICs between amp and preamp. Regardless of the preamp location, I believe it best to have the amp(s) close to the speakers, so speaker cable can be short -- especially with tube amps which need to be close to the speakers in order to deliver as much damping control as they can deliver. But this means the amp to preamp ICs could be long, so you want those to be the next quality acquisition.

Next, the sources. Good PCs (first) and ICs (next). Why PCs first? So you can then confidently evaluate different ICs. Some tips: PCs for DACs or CDPs should be 10 AWG, and shielded. Also, using balanced (XLR) ICs between DAC/CDP and preamp seem to give best performance, if your components allow that option.

Phono cable from tonearm to preamp (or to phono preamp) next, if you have analog. Especially important not to "cheap out" on these if you are using a MC cartridge with its inherently low output.

And last, speaker cables. Oh, they're very important, but you can evaluate all the other cable selections with comparative accuracy even using mediocre speaker cables. However, trying to evaluate speaker cables before optimising the rest of the system cabling is IMO an exercise in futility ;--)
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I think of it like a tree with trunk, branches, and leaves. All the energy passes thru the trunk, the power cord, as Robert points out. Next most energy is thru the amp/speaker interface, and next most the preamp/amp connection, and last (the "leaves") between source/pre-amp.

The speaker cables are sort an equivalent "branch" in the hierarchy, to the amp/preamp connection (granted they carry more energy.) So one could theoretically make a case for upgrading the speaker cables right after the PCs. However, I'd argue that even decent gauge but inexpensive zipcord would allow accurate comparisons of cabling on the "input" side of the amp, and because they're not part of that "chain" of components/interconnects, that leaving them for last, after improving everything else, would allow for better hearing comparisons of different speaker cables. Just a thought ;--)
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Mike, you must've read my post about discovering the (unused) 20A garbage disposal circuit in my apartment. There's now a 20 foot 10AWG umbilical running from under my kitchen sink to my ExactPower regenerator :--)

But I have to agree w/ you about the overwhelming gains from implementing a ded. circuit. Even in an apartment, if you can "latch on" to a circuit that is 20A instead of an ordinary lites 'n outlets 15A ct., it can mean a big improvement. In addition to a disposer, you can often find single purpose 20A circuits for microwaves, dish washers, washing machines, and even hair dryers (unfortunately in the bathroom) or steam irons in the ironing board closet. Make a good umbilical to supply your system and just don't use the microwave (or washing machine) while you're listening ;--)
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