I'd like to add a few words about cables while we're talking speaker impedances. Some things get overlooked.

[u]NEVER[/u] use a guitar cord to connect an amp to a speaker cab!!!

Guitar cables are designed to carry the signal produced by a couple of pickups. A set of active pickups with a preamp turned up full might kick out a couple of volts. The impedance of the amplifier input circuit would limit the current flow to much less than a milliamp. Passive pickups produce even less.

Raskolnikov's amp is an extreme example, but the numbers are fun to play with. 1200 Watts into a 2 ohm load implies 49 Volts at 24.5 Amps! To deliver that power to the speakers you need at least #10 AWG wire. Anything smaller will just piss away those Watts as heat. This would not only be frustrating, but also potentially dangerous. A guitar cable wouldn't have a chance! With very low impedance speaker systems, the cables can become a major part of the total load. And you [u]don't[/u] need shielded wire!

Don't get led down the garden path by some sales-type with a set of high-priced 'golden' cables. For anything less than 200 Watts, a twisted pair of #14 AWG wires less than 20 feet long is ideal. For lower power, or shorter runs, a #16 AWG 'lamp cord' is just fine.