This site is funny this way at times.........unless you own an MC, CC or BU and are loading it with 10K worth of Pro Audio, your not worth a response from some peeps.
I doubt the footrest is a structural wall, so cutting a sub hole should not be a problem. Do you have access to the back side of that area in order to fish you wire? As long as it's not the stringers behind it, you should be ok.
Do you have a sub and amp already, if so, what are they.