Is there a reason that you can't just split the original output and run directly from the 1070 to both subs? The pass-through would hopefully be transparent, but I know nothing about the specific subs in question and so don't know if that is the way they are designed. (There could be something else in the circuit, like a crossover network or such, that would have an effect.)

Also, I would expect each sub to have different output levels when their volume knobs are set to the same position - there are too many differing factors (amplifier being used, efficiency of the driver and its respective enclosure, placement in the room, ...). You should probably adjust their volumes by running each by itself and adjusting the volume controls so they achieve the same output level.