get the thiele.... thump of closed back.... spread of an open back except no volume loss because its front vented.... they're just as loud as a 2x12.... great with the mark iv
Is this the only cab you'll be using? A vintage 30 isn't really rated high enough to be used alone with a IV. A Black Shadow (in a new Thiele) or an EV 12L (either in a used theile or retrofitted) can handle the power output of the IV.