Do Banana Plug Adaptors Degrade SQ?

In theory I suppose everything de grades.  That said how great was your previous connection? How often did you open it and remake it with a clear wiping motion that cleans oxidation? Getting out of this minutia I can tell you that I can get absolutely superb sound with lots of connectors so long as they are tight and clean my two senses people worry about the wrong stuff

Not without soldering that I know of, that is what I do with short OCC wire, or one time with Kimber. An extensive search might turn up something, probably bigger and worse sounding

I’m using them on my setup running from a McIntosh MC2205 to a pair of large advents. Clear as a bell. 

This seems to be what you'd like, but I can't discover how it was achieved.

http://audioklassiks.de/wordpress/?p=2303

If you aren't comfortable soldering and opening equipment it might not be possible. Removing speakers to access the inside of a cabinet can be distressing and extreme care needs to be taken not to damage delicate cones, so I get you concern, particularly if slotted screws were used.

Having said that, it is entirely possible by encasing the screw head with your thumb and forefinger to keep the screwdriver in contact with the screw, and using a very long screwdriver, which helps keep it perfectly perpendicular. Just use as little force as is necessary to keep the screwdriver in contact with the screw and slowly rotate the screwdriver. Gently and slowly is the key, one screw at a time. If your muscles are up to it, consider taking a few goes for each screw, so there are no mishaps.

I have used the cheap banana to spade adapters from Amazon and I thought they were fine until I got a set of XOX Spade Adapters from Black Cat Cable.  
The XOX adapters not only worked better and had a better construction, they sounded essentially perfect as if there was no adapter attached.  I use them to this day and will until the day I depart this life.
Just my 2 Cents worth.