Can these violins be used without their control box, i.e. just plug the violin directly into my amp/pre-amp pedalboard (just like the YEVs)?
The control box for the Yamaha appears to be a pre-amp with EQ, DI, and headphone amp. To be honest, it looks like a pretty good addition to the instrument.
To answer your question, The instrument has two piezo pickups in it and the output signal should be the pickups sent through a blend circuit, which would let you plug it into any amp, pre-amp, or effects unit built to take instrument input.
There are other manufacturers of D.I. and EQ units designed for Piezo pickups, and you can build a completely different tone to the instrument using different combinations of pre-amp and effects.
I have a cheap electric violin that I found sounded much better when the built in pre-amp and EQ unit failed and I wired the pickup out direct.