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

i go bare wire when i can... i also go commando ;)

seriously, any decent made banana will do... just make sure contact is good

@ johnspain
As the poster above said "go commando", I agree!
For good sound quality go bare wire, tinned at both speaker and amp ends. Make sure both ends are very tight which is more important to sound quality then any connector will be.

Bare wire here too 

Tinned with Cardas solder.

Bare wire unless you want to change something.. Most folks know different terminal ends and HOW they are attached make a pretty big difference in SQ.  Especially if they are loose and open air.  Time will change that connection. Silver gets better, copper gets worse..Gold stays close to the same.. (contact wise over time)

Second, different material and plating.  Solder or secure with screws and tin or NOT to tin the ends and just tighten the screws. 

Did you dip the bare wire and the terminal end after a solder or screw tightening, in contact enhancer?

It all matters/changes the sound.. Then let it settle for a while..While it's breaking the rest of the way..Pure silver wire, holy cow... 400 plus hours, for anything  #12 or smaller.  Copper, drop it on the floor, 50 hours..

Little secret. NEVER tin the ends on silver wire. you'll never enjoy the wire if you do..Graphene paint the bare wire and tighten the lugs on mine. 1/2" silver lugs.. to the internal pure silver S/C Teflon wire inside.. The bass section, I don't use. But they are wired in all 1" flat braided copper in a loose silicone tube..Very caveman looking..Huge solder joints..Very polished to this day..The copper looks pretty good after 12 years.

Regards