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

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."

I tried many different connections, but by far the most natural and real sounding speaker cable connector is the ETI Research pure copper spade. I always solder and use Wondersolder with very good results.

These ETI bare copper spades are better sounding than Furutech gold plated FP-201, even when you solder them instead of clamping with the screws.

I don’t like “squeezing” a wire into a connection ( unless it is a thick solid core like AWG 16 or more). The thin wires will be squeezed and this is limiting the “electron flow” since the surface is damaged. 
And please DO NOT TIN ( solder) bare ends and put them into a screwed connection. This will not give a solid connection as the tinning makes the cable very soft and is flowing under the pressure of the screw and being a loose connection.

I purchase the ETI spades from Hi-Fi-collective.uk, as it is half the price compared to VH Audio.Hope this helps to answer your question.

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.

For the most part - I agree with this statement

Some banana plugs have little spring clips on the pin
https://www.amazon.ca/Monoprice-109436-Plated-Speaker-Banana/dp/B0097JLQVC

Some banana plugs have a little bar on the pin
https://www.amazon.ca/GearIT-Premium-Braided-Speaker-Plated/dp/B0713ZDBM2/ref=redir_mobile_desktop?i...

Some Banana plugs rely on the spring effect of the pin itself to hold them in place - BUT they are generally NOT perfectly cylindrical, but elliptical.
https://www.amazon.ca/CERRXIAN-Speaker-Locking-Insulated-Connector/dp/B07TYK9SHQ/ref=sr_1_7?crid=3FV...

Some banana plugs have a screw lock that widens the pin
https://www.amazon.ca/Copper-Nakamichi-Speaker-Amplifier-Connector/dp/B072F1YRT7/ref=sr_1_17?crid=3F...

Many plugs are gold, rhodium or even platinum plated, but the issue here is - these metals requires a substrate to adhere to the base metal - that substrate normally has a much lower conductivity than the metals being used - so it impedes the transfer of the signal.

All of the above designs are flawed - they all fail to make use of the maximum possible contact area

However, the Banana plugs from KLE Innovations are precisely engineered and are perfectly cylindrical. They do Maximize the contract area of the entire pin - which in their case is approximately 1.1 sq cm. That’s possibly the biggest contact surface area of any speaker cable connectors - spades or bare wire!

https://kleinnovations.com/kle-innovations-klei-products/klei-harmony-connectors/klei-harmony-banana...

They are also NOT made of brass or some copper alloy - they are pure copper with a silver plating - so conductivity is maximized

BTW silver applied to copper does NOT requires any intermediate substrate plating material.

But they are not without any shortcomings
- they "may work loose" - IF the speaker cable is heavy - 10 gauge max
- they are a little fragile - they can bend out of shape IF you are not careful
- they can only be soldered onto the wire - some prefer crimp/screw style

Being silver plated they can "tarnish" and some people feel the need to clean them.

But here’s the thing, that dark discoloration is actually SILVER SULPHIDE
- Silver DOES NOT actually oxidize - it reacts with Sulphur in the air

And here’s the thing with Silver Sulphide...
- it does conduct electricity, but not quite as good as silver
- it is only ever microns thick
- restoring full performance is as easy as reseating the plug - i.e. "IF required" 

The KLEI Bananas are so well engineered/fabricated that air seldom gets into the joint, so the tarnishing effect on the pin is almost non existent.

If the plug gets deformed, restoring it’s shape is very easy.

They may not be for everyone - BUT if you want to improve the connection of your speaker cables - give them a try

They are the best Banana plug I have tried in the last 4 years

Here is my review...
http://image99.net/blog/files/tag-banana-plugs.html

Regards - Steve