X-terminators add an interface level and, while convenient, slightly veil the sound.
The AQ binding post tool is the goods - highly recommended.
If you cannot tighten them even with a socket or the AQ tool, something may be broken as Ncarv points out, or you may have to get inside the speaker to tighten the nut on the crossover side of the post - had to do that on my DSigs when I first got them.
