The Balance control on the face is a pickup/synth blend control, and Level is an overall output volume control to adjust when changing the Balance.
The Future Impact is a mono-synth comparable to a typical Moog, I doubt anyone buys a Moog and complains about its lack of polyphony.
Show me anything comparable to what the Future Impact does (full 4-voice actual synth controlled by a string instrument with full onboard and hands-free hardware access to all internal presets with no need for a separate controller pedal) for under $400. Polyphonic tracking at the speed of the monophonic tracking in the Future Impact is also non-existent for bass on the market at this point without using a hex pickup (individual outputs and processors per string- and the guy that designed the Future Impact is one of the pioneers of that technology).
If you want cheaper, with polyphony and comparable tracking, then you probably should look into the Boss SY-200 (or SY-300 for a little over $400). It's not an actual synth triggered by your playing, but instead is manipulating your input signal into something resembling synth waveforms and so doesn't have latency required for pitch-tracking purposes. It does have a sort of glitch at the attack of notes while it pulls apart your input signals into individual notes though, and lacks the depth or tweakability of an actual synth like the Future Impact. Otherwise I think your expectations are beyond the current technology...