To me it sounds like you have the master volume way up and are using the input volume to control the amp volume. You should try to get your sound into reaper with your master turned all the way down (for now).
You should be able to hear your bass through the monitors, which is really what you should be listening to anyway.
Starting from scratch:
Amp off, plug DI out (xlr) into XLR in (phantom off)
Master VU all the way down, EQ flat, Gain all the way down.
Plug your bass in and turn on (don't expect to hear anything though:))
While slowly turning up the Gain, check to see if anything is coming into Reaper.
When you've got something, switch pre/post and see if it's the same. It should be.
Play with EQ and see what you get.
If this worked, you should have lovely clean bass coming out of your monitors and nothing from your amp. You can then, SLOWLY, turn up your master, though really if you are recording, you should just listen to your monitors.
Hope it helps