According to the product manual, among the the recommend processing for the VQ series speakers is a 24dB/octave, Linkwitz-Riley high pass filter at 400Hz or greater for the VQ DF boxes and a 48dB/octave, Linkwitz-Riley low pass filter at 500 Hz for the VQ MB. So there should definitely be a crossover around 500Hz between the two boxes.
The VQ 85DF in passive mode is rated at 8 Ohms and 200W Continuous/400W Program with a 110dB at 1W/1m sensitivity. The VQ MD is rated at 4 Ohms and 1,000W Continuous/2,000W Program with a 105dB at 1W/1m sensitivity. The C 28:4 is rated at 700W into 4 Ohms or 8 Ohms. I guess you could use the selectable Voltage Peak Limiter to limit the outputs for the DF channels but unless they don't need the full output of the VQ MB that seems to be a somewhat unusual amp choice for either box.
Is there any system drawings or documentation? If not then it sounds like it might be a good time to create some.