It's a matter of current capacity on cables. If (as on each set of my subs) I'm running 3 kilowatts into 4 Ohms, then P=IsquaredR so I=root P/R

So (if everything were DC which it isn't, but it makes life easier) current is getting on for 30 amps.

The biggest 2-core mains extension cable is under a third that.

Housing cable, in up to 30 amps (twin and earth) is very good, but it is a bugger to work with being so stiff, and very prone to fracture. Almost no capacitance or inductance losses both of which can be problems with brads or twists. I do use it (properly supported) inside speaker cabs.

The Maplins HiFi cable has a nice big cross section which I'd trust to about 20 amps and is fairly cheap. I buy a reel every time I see it on special. The real mcCoy 5mm (square mm)armoured sleeved speaker cable costs the earth, is too big for most connectors and won't go throughthe hole in many connector covers.

THe Maplins hi-Power cable is thinner, don't go there.