Thanks folks!
I roughly figured about 4.7 cubic feet going by external dimensions minus 3/4" of material per face and recessed grill. Didn't go as far as to account for bracing (probably minimal!), volume of port tube or drivers. I'd guess from that that it's maybe slightly more than 90 liters, but probably not much more than 100, 110.
I'll unhook the crossover first and take it from there as suggested.