Since it was a guitar cab, the port tuning may be a little on the high side for bass and the noise could just be you reaching that resonant freq.
All the other things (VC rub etc) are also possible.
Take the speaker out of the cab and play through it - if you have another cab you can put it in then that's ideal, but you can still hear VC rub or cone fatigue without an enclosure. Do you get the same issue? If not it is likely the port tuning (replace with a wider and/or longer port; or add a second port.... the internal volume may be small too, you can try stuffing the cab to make it appear larger to the speaker).
The buzz getting louder depending on cab orientation can be caused by many things, including room acoustics/geometry (how the lobes of sound from both drivers interact in the room; additively and/or subtractively) or because the VC rubs more when oriented one way vs the other. To test that, turn the drivers 90 degrees in the cab and repeat the orientation test.
Note, I suspect that this is port/cab tuning because the resonant freq of a guitar 12" is 65-80 Hz usually, so a 70Hz C is right about where I would expect the port tuning to be. It may not sound like typical resonant peak because it is not the speaker's resonant frequency, but the cab's; higher then the speaker's and more energetic (because you're playing a bass not a guitar). The speaker cone may also be buckling a bit (cone breakup; can sound kinda like distortion or fartiness, depending). If the cone breakup is this bad then it could be time for a recone (all bass drivers will fatigue and need to be reconed depite lack of VC issues eventually; you will often be able to see creases or stress marks on the cone, especially if smooth vs ribbed.... super rigid synthetics and metal cones possibly excepted, but assuming standard paper or poly cones, especially paper).