...and carpeting won't work, bass frequencies easily penetrates those materials...
...and as others posted, the only way to soundproof a room is to prevent the room itself from vibrating, which by itself becomes a large transducer!
I'm not sure any of the effective solutions are going to be cheap or quick, and your best bet is finding a remote location to rehearse...
...and NO, you won't be able to power 3200W worth of Ampeg and Marshall heads and a Mackie board through the storage unit light bulb socket!
