Audphile1, it wouldn't cost anything but a little of your time (and forgive me for that if you're a $400/hr. attorney!) for you to call or email the makers of all the cables you compared to get the actual specs, particularly shielding topology (connected at one end or both ends?) and especially conductor size. This would be very helpful to you in correlating what you're hearing to the construction of the cables you're using. My personal experience with amps and DACS has been the bigger the conductors, the better the performance FWIW.
Clio09, I agree, the 8 GA. is definitely overkill for a phono preamp, whether tube or SS. 14 GA. should be more than enough. Your comment "all appear to be shielded at both ends." is a bit confusing to me. Here's how it brakes down with respect to shield topology (layout):
1.) One shield (or two, like a braid over a foil, but acting as one) connected to the ground pin of the AC plugs at both ends. This is true for OEM supplied shielded cords because of UL regulations. However, by being connected at both ends, the shield can carry current. This arrangement creates two problems: a.) ground loops (hum) and b.) the shield becomes an additional (unwanted) ground conductor leading to electrical problems (capacitance/inductance) that can affect sonics.
2.) One shield (or two together as above) connected at only one end (floating). Most manufacturers connect the shield at the AC plug end, but some connect it at the component end (I don't know why and wouldn't buy one of those myself.) In this arrangement, the shield can't carry any current (no point-to-point connection) so it simply acts like an antenna attracting RFI and getting it (draining) to ground before it can add itself to the hot conductor. Or, with digital equipment, preventing the hot conductors (bottling them up so to speak) from broadcasting digital RFI produced by the component's power supply, into the air.
2a.) One variation is the "double floating" shield (Cardas uses this I think, and maybe some others like Virtual Dynamics?) Here, there are two (usually concentric) shields (like the foil and braid mentioned above) BUT they are insulated from each other and do not act as one. Instead, one is connected to only the AC plug, and the other is connected to only the component plug. So they're both floating, but from opposite ends of the cord. I've never (to my knowledge) used one of these -- sounds good in theory. Might be overkill for audio applications, I don't know.
3.) Shielding that isn't connected at either end. You can think of this as a ferrite bead or donut that has been stretched out or extruded to be as long as the cord itself! And just does its work by the sheer absorbing power of its material. A well-known example of this would be the Purist Audio Design "Ferox" shielding, a thick layer of powder or paste that surrounds the conductors. Another manufacturer uses a jacket of metallic beads to do the same thing.
