I would consider swapping the binding post on the amp. As you already know, the connection at the amp or speaker is of major importance, and no matter how high the quality of banana/spade adapters you will find, they still will not be as good as high quality binding post and spades. Also, in absolutes, a binding post will always be better than a banana of equal quality.
My recomendation for a better converter is no converter. While I appreciate the fact that high quality converters are available, so are terminations (bindinig post, spades, bananas, ect.) This is a rare occasion where you can get a free, no-brainer upgrade in sound quality, so why not? (what is the cost of new binding post as opposed to adapters?)