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

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

These are the desirable things in a speaker wire/connectors:

Must have low resistance so as to not increase the output impedance of the amp. You get that by using better and thicker conductors. Copper is fantastic. So, the purer the copper, the better the conductance. Having said that, no need to be ridiculous either. 99.99% of the people wont be able to tell the difference between 99% pure copper
and 99.9999% pure copper. Not worth the extra which only a very sensitive measuring device can notice.

Lower capacitance. Higher capacitance will impede hi frequencies. But then again, you
will not notice it in a few nano farads.

Lower inductance. Do not coil your wire, run it straight. Increased inductance may affect the rise time of your signal due to an inductor resisting the rate of change of current thru it.  Again, a few nano henries will not be noticed.

A perfectly good pure copper speaker wire of about 12 gauge can easily be bought for about $50. No need to spend any more, unless you want to run miles of it.

Low mass gold plated Z plugs.

Very tight fitting for maximum contact area. You can stack them into each other for jumper cables. They are not expensive.

Contact enhancers worth doing get a graphene one if you can it does make a difference. Dampen with fo.q. ta32 tape.

I wonder if they have done significant blind testing?

I would also be curious to know if I were blind tested to compare speaker cables if I would hear a significant difference?  I doubt someone would sit down to listen to a system and say this sounds like you are using Tera Labs or Kimber Cable.  I do think buying a decent set of cables does make sense, but not spending more than $1,000.  However, $1,000 is a lot of money.  The system should I guess cost at least $10,000 to warrant the expense.

Banana plugs are great for experimenters and folks who make a lot of changes, but they don't offer much contact area with the barrel they fit in to. You want maximum contact area. After all, there are two ends to the wire and the net effect of both connections must be considered. While I use many banana plugs and buy dozens every few months, I know spades or bare wire will give me a better connection. Remember, bare wire will squash down and fit the contacting surfaces very well. Copper is rather soft and will flow over time, loosening the connection. (That's why there are electrical fires in old houses.) Tighten them up every year or so. Spades seem to offer a good connection, but how do they attach to the actual wire? You have two connections on each end! The wire to the spade, and the spade to the speaker terminal (not to mention the terminal to the lead that actually goes to the actual speaker!). So, considering there are two connections inside the speaker "box," it is best to use bare wire and tighten the connecting mechanism really tight. The "tinning" you hear or read about is just to keep the many strands of wire in many types of cable from separating as the connections are tightened. A good idea. Use "silver solder."