Opinions vary on all types of connectors. Some swear that spades are the ultimate.
I use bananas for simplicity of use and notice no difference over bare wire or spades. But then it takes a true audiophile ear to take measure of these things...and apparently I don't have it.
No improvement. I use them for every connection as well, just because there's no chance of stray wires and it makes connection so much simpler to use. How long does it take you to twist the wire ends, insert, then twist the connector for each wire vs just popping the plugs in?
There was a thread not too long ago about someone having problems with their system that turned out to be a few stray strands of wire. Someone's comment was that bananas show their value in a situation like that.
I don't like spade connectors, and they really make no difference compared to bananas.
Spades are superior due to having the largest contact area of the bunch. Whether that translates into an audible difference is debatable.
I use both in my system. The harder to access connections get bananas for ease of use & spades on everything else. Bare wire is a pain to deal with, especially on some of the amps/receivers out there without true 5 way binding posts.
I too have used both. The only drawback that I find with bannana plugs is the amount of extension they cause behind the receiver. They stick straight out where as spades allow you to exit down at a right angle. Either way, I prefer to solder my wire to contact connection after crimping. I then follow up with shrink tubing over the joint. It's easy for me, I work for an electronics manufacturer that does alot of assembly work.
The bananas I use let the wire hang straight down, but you still need about an inch behind the receiver, which isn't a big deal for me. Marantz gear is designed so you cannot use spades.
If there is no audible difference, there's no serious argument for using spades over bananas or vice versa, it's just a preference. I've had spades loosen up and lose contact, but I've also had bananas that didn't fit well in an odd sized binding post.
I used to solder my connections too, but found that it really didn't add anything except strength. If you aren't using silver solder, you can actually degrade the sound slightly.
The bananas I use allow the wire to hang out from the side, so strain relief isn't an issue, but you do still need about an inch beyond the binding post behind the receiver (not an issue for me).