Speaker terminal Loose

Are your Furutech bananas the kind with the split hollow tube at the end?  If they are just get a small screwdriver and open them up a bit. 

Millercarbon.   You are correct about sacrificing spring action and sonics.  Having a pure copper base is always better than less conductive brass.  But copper is soft and difficult to machine.  That is why you still see most "high end" speaker connections and RCAs made with some kind of plated brass.
The only banana plug I know of that is pure copper are the Furez connectors.   They are soft but they have a plastic insert that helps with holding them to shape.  They grip good but do need an occasional expanding when they are plugged in and out a number of times.  I have not noticed them losing their grip if they remain plugged in.
I do like spades.......just wish they would fit on all of my amps.

You can’t always blame the banana plug. Different manufacturers used slightly different binding posts over the years. I have gear where the same plug fits tightly in one component and loose in the other. Some plugs also fit deeper in some of my speakers. While close, there is no industry standard on the exact dimensions and there is nothing wrong with doing some minor fitting to get a tight connection.
I have tried locking plugs like the WBT.  The knockoffs I tried were made with extremely hard brass and I cracked one when I accidentally bumped it.   I am sure WBT uses a better alloy but it still has to be a relatively hard brass alloy.    I also became concerned with the amount of contact area with some of the expandable plugs.   If you out it in on oversized plug and expand it till it is tight........what is the actual amount of surface contact?  


I have had better luck with the BFA syle banana plugs.