The Direct Out is intended to be a replacement for miking the cabinet, so it needs to go to a mic input. Some powered speakers (QSC K-series) have a "mic" setting which we used to set the level and voicing of the Direct Out.
Going to a Line In will be weak.
--The FX Send is a 1V peak signal that should drive a powered monitor OK. It will however have a "raw" unvoiced sound compared to the Direct Out, but it might be OK for some purposes.
--Add a small cheap mixer from the Direct Out, to its mic input, to your powered monitor, if it doesn't have a Mic setting.
--You will notice that unplugging the MP speaker, while safe, affects the sound of the Direct Out by unloading the amp output. It sounds best with the speaker connected (The FX loop is not affected by this factor).
--A passive extension speaker could be used, positioned so you can hear well. Our BlockDock 10 tilts back for this purpose.
--If you are mainly going thru the house, maybe you can use the amp itself as a monitor, get the volume you want, and then set the house level. The house level will change however if you turn the amp up or down, just as if you were miking it.