Yep, the treble bleeds/fidelity widgets are shorting your hot to ground when you turn down either volume. On an LP style it doesn't matter as the middle lug (out) is shorted to ground when turned down. On a normal bass with 'independent' volumes the pickup on the middle lug gets shorted, but with the treble bleed attached the out is still linked to the middle lug... so when you turn down the volume it becomes the path of least resistance and your signal goes to ground. Remove them if you don't have a pickup switch.