on the RUmble, the Effecs Loop SEND goes to the INPUT of your Zoom, and then take the mono out from the Zoom to the RETURN of your Rumble.
this excerpt is directly from the manual:
FX LOOP — Connect the SEND to the input of outboard
effects devices (delay, chorus, etc.) and the output of the
effects devices to the RETURN. Placing effects in the
FX LOOP (instead of between your bass and the INPUT)
will reduce the amount of noise and tone degradation
caused by the effects pedals.
TIP: You can connect the SEND jack to other devices
(such as tuners, or additional power amplifiers) without
connecting signal back to the RETURN jack. Returning
signal to the RETURN jack is not required for operation
of the amplifier. Additionally, you can connect external
signals to the RETURN jack to bypass the preamp controls,
and send external signal directly to the power amplifier