It's a bit of a balancing act, but what I do is have the Blend and Level set on the pedal and the Gain set just so on the amp so that the volume appears to be the same when the pedal is on or off, only you get a bit of grit from the gain when the extra signal from the pedal pushes it over into distortion.
You can do much the same thing by setting the Gain where you want it for distortion and backing off your volume control on the bass a little so it's cleaner - then whacking the volume control up full for the grit when you want it. Then you don't have to buy a pedal... It's a bit trial and error from the point of view of not having too much of a volume increase, but once you've figured it out, that's it. The key is setting the Gain at exactly the right point so it's acceptably clean when you want it to be, yet doesn't take too much extra signal to push it over.
The joy of the Streamliner is you can set your Gain and Volume just so and use the Master for overall control of the level. I don't know why more amps don't have Gain > Volume > Master, but they don't!