Do the SPX series connectors interchange with the original Speakon connectors? They kind of look like the do, but I can't tell for sure.
Yes. The SPX is a higher powered version of the basic connector, and the STX is a metal variant of that (basically armoured). They all have the same physical config and will interconnect with each other. Myself, I have been using the SPX series stuff on my bass rig, and it is great . . . . Unless you are touring a large PA, or have an exceptionally inept crew, I can't see the need for the STX, especially since it has the same current rating as the SPX . . . . but your situation may be different. - Tim