Tech 21 makes a bunch of Sansamp units you might be using. I'm guessing that you're talking about the BDDI because it's the most common, but if you're using the RBI or VT, you should let us know.
I have a love/hate relationship with my sansamp.
I have had my whole rig centered around it. Now it's my overdrive pedal, and is off most of the time. It's at the end of my pedal chain and I DI and OD with it.
The sansamp has a very direct and proportional relationship between "Grit" and "hiss."
Moreover, it will sound different depending on it's position in the signal chain. It takes a lot of experimenting. You have to twist those knobs. Any reduction in the Blend, Presence, Drive, or Treble will reduce your hiss. Make small adjustments to all of these til the unwanted noise starts to disappear.
Give it a HOT signal. Get your other gear with a cleaner signal as hot as you can get it. Sansamp likes a hot, clean signal.
Might be best in your effects loop, might be best between your bass and your preamp. Might be best between your preamp and power amp. Give all the locations a whirl.
And toss those presets from the manual out. This thing is waaaay to fussy (IMO) to have any of those suggested settings remain useful given all the different uses one might give the BDDI.
Have you flipped the Ground Lift switch? You might be hearing a 60 cycle hum from your power.
Just experiment. That's all there is to it. I'd be willing to bet you can find just the right settings on the unit, your other gear, and location in your signal path to be really happy with it.