Rob,
It is true that signal strength decays as the cable gets longer. Also, when you have a long stretch of wire it acts as an antenna for radio frequencies. So, a really long instrument cable can introduce noise (via RF interference) or signal loss.
I don't think I would run a standard instrument cable longer than, say, 30 feet for fear of the 'antenna factor'. Naturally, how much RF you pick up is dependent upon where you are. It's probably much more likely to be a problem at an outdoor gig than in a basement.
For really long runs, if you don't go wireless, you can use two direct boxes with a long microphone cable in between. (a short instrument cable from the bass to a direct box, then a long mic cable [same gender on both ends] to another direct box, then a short instrument cable from direct box to amplifier)
But that starts to get a bit impractical.
The instrument cable is has a single wire with the signal on it, and a shield. Mic cables have 2 wires that each caarry the signal, but the signal on one wire is 180 degrees out of phase with the other. This allows for the cancellation of any noise or hum introduced along the lenth of the cable, thus yielding longer cable runs with little or no noise.
My current favourite is 5 metres long with a right-angle at the bass end and a straight at the amp.
Really weird!
