Hmm..dont know all about that I mean you might as well get a hammer and bang the ground...but it feels like concrete,asphalt,etc has a resonant freq rather high like 45-60hertz to feel it vibrating.
I was referring to the first pic because the bolt/post look a little small for the ring terminal but if that's the only place then just leave it there i guess...
Try that song "Woofer Cooker" by Boston Acoustics (on their test cd) it goes from a freq sweep from about 100hz down to a constant 32hertz,should slam on your 15's man.Also anything Young Jeezy lol.