Just got a Evidence Siren II speaker cable, 3 ft long.

Their description: Open and extended midrange and high frequencies without glare or etching. Preserve clarity and focus, revealing harmonic detail and articulation. Increases space between conductors to reduce magnetic interaction A bundle of six 20 awg conductors which avoid skin-effect anomalies yet large enough together to provide ample damping. Holds conductors rigid to minimize electro-magnetic modulation of the conductors. Delivers a dynamic and dimensional presentation. Provides flexibility while protecting conductors

My Setup:
Mesa road king II series II head
Mesa Rectifier 2x12 cab
Epiphone Les Paul
Planet waves guitar cable

My comparsion: I've been using one of the cheap speaker cables that guitar center gives you when you buy a cab there. The cable has neutrik ends, which seem sturdy. The cable is also unidirectional, with one end having a red cap. The actual cable is pretty stiff, not lots of give.

With only a few minutes of going back and forth between this cable and the Siren II, there is a definite difference. On clean channels, the bass is more defined, tighter, louder. The higher notes are less nasally. Striking chords produces clear definition. On distorted channels, bass notes are more defined, less mushy. Distorted chords are also less mushy, more defined.

It says on their data sheet that the "break in" period is 40 hours, but that 80% of the break in occurs after 10 hours. Would be interesting to see the change after that.

Also, according to another review I read. The effect of evidence cables increases with each cable you replace (speaker, guitar, power cable).