Gregadd,
Setup caused the problem you experienced with a single sub. A single sub, properly setup, should disappear as well. But setup may be easier with two subs. Don't know about your subs, but I have mine turned down to a quarter scale at the sub and use the pre attenuator to match the 5.1 levels with white noise and my RS SPL meter. It can generate that felt vibration with a low pedal note or make you feel your chest is compressed with an explosion in a movie. So I doubt adding power is the reason for a second sub.
db
Setup caused the problem you experienced with a single sub. A single sub, properly setup, should disappear as well. But setup may be easier with two subs. Don't know about your subs, but I have mine turned down to a quarter scale at the sub and use the pre attenuator to match the 5.1 levels with white noise and my RS SPL meter. It can generate that felt vibration with a low pedal note or make you feel your chest is compressed with an explosion in a movie. So I doubt adding power is the reason for a second sub.
db