You don't need the seal and you need the range helm. The stats are invisible, but it adds a % to your range damage and accuracy. It's the only item in 07 that adds DAMAGE, not accuracy. Dhide only adds accuracy, that's why void is so good.
Just in case you want the exact percentage boosts -![]()
Source: http://2007.runescape.wikia.com/wiki/Void_ranger_helm

