Well after looking at the manual, it does seem like you're lacking a lot of what I would consider normal level management within the unit. But still, it would seem that you could do something in the effects parameters, like adjust the direct level up and down, or the reverb level up and down, etc. If you can add Chorus, and put the Chorus level up all the way, but back down the delay and modulation, you'd have a doubling effect that would compound the volume for solos. You can do the same with Reverb and Delay, by setting the delay time short or to zero. I see what you mean, though. It might be time for a stereo volume pedal after it, or something like that.
I'm sure it sounds great and for $200 it's a bargain, but my old Quadraverbs can do 10 times what that can do and they're selling used for less. They have internal mixers at all the stages of the signal path. That's why I assumed this MV4 would have to have something in the software to be able to adjust the direct level, or master level, etc.