The SubPac S2 system is designed for a wide variety of chairs. Most users will not be able to detect the differences going from chair to chair, but if you are using the S2 for hyper critical listening its important to understand how the chair interacts with the subtle nuances of tactile sound. With a little experimentation you can reduce or eliminate these artifacts from the system. Because of the damping nature of some chair designs, materials and form factor, some adjustments may need to be made. Generally the user should select a chair that is comfortable and can accommodate the full height of the S2 with some room for body positioning. The padding on the back of the chair can influence the system by absorbing certain frequencies. Find a chair with medium stiff padding. Avoid completely solid backs (or put a towel on them), and avoid overly soft or fluffy chairs. Most office and studio chairs meet these requirements.
Some chairs can resonate at very low frequencies. Most of the time this happens with very light chairs when the SubPac is connected to the seat and the back at the same time. Try adjusting the strapping system higher so it is only touching the chair back. If the chair still resonates at one particular frequency you can add weight to it or try setting it on a rug or carpet.