This isn't about instrument amp to speaker cables, but is related. A few months ago, a friend built me a 3 watt per channel single ended triode power amp for my stereo, telling me I would be blown away. I doubted him, but once it was done, I was blown away. The level of detail and the soundstage are incredible. That led me to a search for the right speakers. I finally put together some boxes with Fostex full range driver (since I needed something very efficient, ie. no crossover, and with a very benign impedance curve). My next search was for speaker cables. I came across a website with the idea of using CAT 5 cable. It is solid copper and in pairing up the cables, you can end up with a large effective cross section, without the problems caused by stranded cable. I was again blown away! I have been toying with the idea of doing something similar with my bass rig. Go to
http://www.tnt-audio.com/clinica/diycables.html and click on the FFRC section for more details.
Edwin
PS Apparently Romex can work well too. Very thick solid copper wire!