The SPL unit is not passive, the master knob controls an opamp so you certainly have more range.
The SMPRO unit is totally passive as you know (no need for power as well)
The switched attenuator has the following values for the 12 positions:
0, -1, -2, -3, -4, -5, -7, -9, -11, -14, -19, -31dB
What I would normally recommend is setting the loudest volume you would ever listen to as UNITY on your interface ZERO on the MPATCH5.1 and reduce the volume on your amp or speakers so that is the MAX.
That way you will get much more control over the level in your situation.