Get a Big Muff. J uses both vintage and modern Big Muff pedals. Look at http://www.guitargeek.com to see his setup. A new Big Muff re-issue is cheaper than the Metal Zone. A Metal Zone owning friend tells me you can get a convincing Muff sound out of it, and it does a lot more.
You can find out more about the Big Muff at http://www.ehx.com