Viggen, the car analogy is interesting, but speaker cables are not like a fuel system, or even a car battery.
I'm not an ee, so I'm going to quote from part of a response I got to a question I asked over at AudioAsylum. I was curious about supposedly directional cables used in a system where the preamp inverts phase and the speaker cables have to be reversed to compensate.
Jon Risch, answering my question at AA:
"these are AC signals, and vary back and forth in terms of which way the signal is actually travelling, neither the hot wire or the ground wire carry the signal more one way than the other (in so far as we know). The signal is alternating back and forth inside BOTH the hot and the ground, without either of which it would not transfer at all.
Both wires are equally important to the signal, and both need to be of the same high quality. You can not just focus on the "hot" wire, and ignore the ground wire."
I'm not sure why you hear a difference in sound, but I doubt it's because you have the "better" wire on the "positive" run. Have you tried just biwiring with the Argents on the bottom and the Midnights on top?