At the moment I am rebuilding a pedal board for someone very special, and I am using Bullet cables and SLUG connectors from guitar to amp.

This came up during the third cup of coffee with a customer:

Q. why can't I use instrument cable to run from my amp to my speakers?
A. because you will stuff up your immediate sound, and eventually blow your amp up.

Firstly there are two electrical concepts that I am going to have to ask you to accept: capacitance and resistance. For now, just trust me, then go look them up.

A good instrument cable needs to have low capacitance, and resistance doesn't really matter. No matter what pickups you use, the current will be low, and the the cable will present little resistance to it. And that goes between pedals, and from your board to your amp. What is of concern is capacitance, and that may cut some of your signal. Cheap cables have high capacitance. How much capacitance in a chosen cable is the question to ask your supplier. If you get an answer like "I dunno", leave the shop and save your money.

Also, instrument cable has an insulated core, with the shielding forming the second "wire".

Now what happens after you run your signal through your 1000W HiFendshallox amp to your external cab?

The speaker cable now has to handle what the amp has turned into a large current, and capacitance is hardly an issue. So now ask your speaker cable supplier what gauge the cable is. If they say 18(swg) or more, or it means that the cable is narrower than 16swg and now is a good time to walk away. That narrower cable will back up signal into your system, building up heat, and eventually may blow your transformer or transistor (depending on your type of amp.)

If they respond to your wire size question with "I dunno", run, run away and never go there again.

Speaker cables are also two "wires", each insulated. Often there may be shielding as well, but unlike an instrument cable it will carry no signal.

Another thing to consider is use the smallest runs of cable you can. Get two identical cables, one 3 metres and one ten metres, plug them from your guitar directly in to your amp, and you WILL hear a difference. The pedal board I am working on is also arranged to have the smallest cable runs, and quality connectors.

Now to upset a few people: the talk of gold plated connectors and oxygen free cable is considered by professional audio guys, including Pete Cornish, to be total bullshi crap. In fact, a gold connector in a steel, tin or iron jack, will set up an electrolytic (chemical) reaction between the metals that will slowly ruin both items.

(If you don't know who Pete Cornish is, go off and be ashamed of yourself)

Moral: get good quality cables of the correct size. You will also notice the improvement in your tone.

Big Note: if you can afford Evidence cables, buy them. I will not lie to you- they are very expensive, but importantly they have a lifetime, no questions asked guarantee. They will pay for themselves in the long run. People with a bit of an ear for tone, like a Mr D Gilmour, use them. An Evidence cable was considered such a necessary part of his tone that David insisted that the Gilmour Fender Custom Shop Black Strats ship with one.

And as far as the Bullet cables and SLUG connectors go, so far they are good. Testing continues. :-D

* I have no affiliation with Bullet cables or SLUG connectors, or Evidence cables. What I use, I have bought.