Hi Michael,
To normalize the systems so that they all let frequencies at 0.1 Hz through without filtering them, you want to ensure that the gain at 0.1 Hz is the same for all systems.You may achieve this by adjusting the gain of each subsytem at the specified frequency.First, you need to find out the current gain of each system at 0.1 Hz. Decide on a target gain at 0.1 Hz that you want for all systems. This could be the gain of one of the systems, or another value that suits your requirements. Then, for each system, calculate a normalization factor.Lastly,Multiply the transfer function of each system by its corresponding normalization factor. This will adjust the gain of each system at 0.1 Hz to match the target gain.
Hope this helps.