Can you be a bit more specific about what is "horrible" about the sound you are hearing? For example, are the high frequencies too pronounced, or the lows muddy, etc.
I did not find that my biwire pair of MI2 cables needed much break-in (although they are only 9' long). The one factor I can think of that might be affecting your cables is amplifier oscillation. The AC speaker cables are known to cause oscillation with SOME amplifiers (such as Adcom), which is why you should use a pair of Zobel networks attached to the speaker connection points. AC provides an inexpensive pair of Zobel networks, but I suggest you make a set using high-quality resistors and capacitors (costs about $25 for the pair). Look in the A-gon archives for the old threads that discuss the R and C values you need.
I did not find that my biwire pair of MI2 cables needed much break-in (although they are only 9' long). The one factor I can think of that might be affecting your cables is amplifier oscillation. The AC speaker cables are known to cause oscillation with SOME amplifiers (such as Adcom), which is why you should use a pair of Zobel networks attached to the speaker connection points. AC provides an inexpensive pair of Zobel networks, but I suggest you make a set using high-quality resistors and capacitors (costs about $25 for the pair). Look in the A-gon archives for the old threads that discuss the R and C values you need.