The headband is made from spring steel, so it's pretty robust and it should be possible to bend it back into shape. I've loosened / tightened the clamp of several Grados in my time.
The best way to tighten it is to first remove the earcups from the C shaped plastic pieces, so you are left with just the headband assembly. Then hold one end of the headband (NOT the plastic parts) in each hand, with your hands crossed over. Now un-cross your hands so that the headband makes a loop shape, with the bottom ends crossed over. Hold it there for a bit and release. You may have to do this a few times, depending on how much clamp you want. If you go too far, you can just bend the headband outwards again.
Note that when I had a 325is, I found that even brand new and clamping hard, they didn't stay on my head very well. I think it's just down to the weight and the on-ear pads.
Clamping force doesn't affect the sound of Grados too much, although if you have them very loose I found the bass can suffer a bit.