[niles]

Jack:
You have some excellent comments.
Since I was in this business of finding noise/vibs on subs; thought I would add a little.
You implied that the noise recently started? That would imply that there is nothing wrong with the balance. If it was a professional built that low probability.
Dscott is on the right track. You probably have a "sound short" ie, something has moved or you mounts are loosening up and your touching metal to metal when you engine going thru the imbalance harmonics. Side note, V8's can't be balanced perfectly; it's dynamically impossible with the V8 configuration.
Go under the car and look at everthing that within . 5 in of the frame. Headers, cluth slave, bell housing, trannie bumps and bosses. Look you any thing shiny or worn and you'll have found your "sound short" They can produce vibs and noises that you can't imagine.
gn