Do you have the treble boost on the bass wide open? That position is known to produce a hiss - though more a constant hiss, even when no note is played.
Assuming the Sennheiser is multi-channel, you might try another channel, see if it's frequency-related.
But you could also be overdriving the Sennheiser: does the hiss disappear when you turn down the volume on the bass? In that case, you could set the trim pot in the control cavity a bit lower, or decrease the sensitivity on the Sennheiser.
The ancient art of trouble-shooting also tells me there may have been something outside of your rig that changed at the same time as you started using the Epics, like something with the lights. That's why I was wondering if you had AB'ed the Epics and a completely different bass in the exact same setting. You say you compared with a Pedulla and a Modulus, but was that really an AB test?
Other than that, the ferrite bead should solve the problem in most cases - but AFAIK Epics have always had the bead installed, so it would be strange if the beads were missing.