About the Slingy 20 bass pedal rock ...
The build has the spurs dead center on the side of the bass and quite a bit up on the shell. I've used just under ten different pedals on them. All of them worked loose as I played. It's all due to the placement of the spurs and my right foot. They just don't work for me. I've used bass stops and even used a level to ensure the bass was right on. Some don't have issues with pedal rock. I did.