Maybe for bass if you're miking the cab, but for guitar this shouldn't be necessary. If the low end of the guitar is rumbling, there's a problem in their rig that needs to be changed! They most likely 1) have too much bass EQed or 2) are overdriving the bass frequencies too much. The first is strait forward but let me explain what I mean by the second. All guitar rigs using overdrive (OD) have a simple chain of EQ > OD > EQ. The EQ either comes from an EQ pedal, tone knob on the OD pedal, the guitar tone knobs, or the amp's tone stack. At a minimum they will have guitar tone > OD > amp tone. With that combo you can chose which frequencies will be over driven more. If you set up the guitar tone bass heavy (and not much highs) run it through OD, then set up the amp treble heavy you'll get a tone where the bass is overdrive and the highs aren't since you gave the OD more gain on the bass. If the bass is floppy find ways to reduce the bass BEFORE the OD and then boost it back to a balanced level after the OD. Should clean it up.

Or if you and the guitarist don't want to change his or her set up, you can move the mike position on the speaker. Closer to the floor = more bass, higher off the ground = less bass, closer to the center = ice pick, closer to the edge = dull.

Skip the isolator though, there are too many free ways to change the sound :)