Sure, it looks like the miniDSP won’t pass enough voltage to drive your amp to max. But that’s only an issue if you need the maximum output from the amp. If you can get as much level as you need from your subs with the miniDSP that you have, then there’s no reason to get another one or a signal booster.
If you can’t get enough level, then a signal booster after the mini DSP will do the trick. But if you're coming up short at .9 volts, I doubt another .2 volts will provide enough extra power to get you from "not enough" bass to "more than enough." IOW, a more powerful amp may be in order.