Ok, the reason I asked is that different amps present different (or no) options for doing this.
Now on the Strategy/Prodigy, it's a little different because both allow you to access the speaker signal in a different way than most tube amps.
- On the Prodigy, you can use any simple passive DI using the slave out because there is a fixed level shifting network built into this output, the result is that you will get standard line level out. Note that you will also need a speaker load on the amp.
- On the Strategy, again you can use a simple passive DI, but the level shifting network is variable so I would recommend starting with the slave out level control all the way down, set your speaker level, then turn this control up until you get an adequate signal.
On other (non-class D and non-bridged) amps, you will usually need to use a DI with a built in speaker to line level pad.
Hope this helps.