Do pure copper banana plugs make a difference in the quality of the speaker sound.

Fact Check @ eric_squires, Gold is malleable / ductile by comparison. 
Copper, Silver, and Gold - Hard to Soft
As @cakids (and others) pointed out can be an effective plating element and conductivity is another matter.

I've posted this before in other threads and it looks like it's needed here;
http://www.tempoelectric.com/Wire_and_Cable_Facts.pdf

It seems using any kind of connector degrades the signal.

All the best,
Nonoise

@nonoise

this is why I suggest low mass, crimped banana connectors with Acoustic Revive contact enhancer. Replicating the force of crimping is near impossible on any normal binding post on bare wire and increasing connectivity of the imperfect banana surface is mandatory. On bare multi-strand wire the likelyhood of breaking wires is high, surface contact is low. On single strand wire chances are that in a bare connection the contact surface is even lower. Soldering adds resistance, no matter how good the solder.

Great reference article, btw

@antigrunge2,

Understood. Though there's so many ways to skin a cat, only a few do it right. (not that I'm into doing that sort of thing)

All the best,
Nonoise