I own a stagg since 2 years ago and mostly played with bow, I use d'addario helicore strings and so far I had no volume issues, although as you mention pizz is far more appropriate for this instrument.
For Arco I crank the second knob in the stagg almost at maximum, I use a Fender rumble 50 and play with the effects it has for a better sound, but just to get a better tone, not volume.
One issue I did find is that strings tend to have a lot of resonance so I put some micro fiber below the strings to damp the sound in the upper side and some sponge below the bridge, I know it sound strange but it has given me fairly good results.
Still strings seem to be not as responsive as if it were an acoustic, it require a little more effort from the bow, not so much as pressure but more "pulling" if you know what I mean.