Impedance plot of speaker cabinets for design of reactive load.

[Humbug]

The Marshall 4x12 loaded with GB+V30 is a 16ohm cab.

I took the liberty of spicing your cab... Maybe this is your excuse to build a 16 ohm load ;-) I'll send you the files (or just the values if you're a masochist) if you're interested. The upper rise portion is exaggerated to compensate for whatever is going on in the highs in my box and will probably need to be changed, and I'm guessing at the actual magnitude and Q of the resonant peak.