OK, since I'm having a lot of trouble figuring this out, I'll be REALLY specific about my chain:
My guitar plugs into a Countryman 85 DI box, which then goes out the "amp" output. It goes from the DI box to my pedalboard (my pedalboard actually has inputs and outputs in it that my pedals connect to). It goes out of my pedalboard to the amp, and from the amp to the cabinet. The rest is in the OP.
I tried all 3 channels in my amp, and the result was the same. I tried a different guitar. I tried moving the mic. I tried a different 57. I tried going straight from my guitar to the amp, with no pedalboard. All with still nothing below 100 Hz.
Also, I bought both 57s brand new from Guitar Center, so they're not fake.
What the fuck.
I have no idea what it could possible be. I just bought the head not too long ago, brand new. The cabinet, however, I bought 2 or 3 years ago, used from Guitar Center. And it came with numerous scratches, and a broken caster. Could the cabinet/speakers be malfunctioning? How would I find out, short of getting a new cabinet?
EDIT: I just ran a spectrum analyzer on the sound going direct from the speaker out of my amp, to the DI box, into the preamp, into the interface, and into the computer. There wasn't a ton of content in the low end (which I expected), but at least if I played a note there was a bit of a spike at about 80 Hz. Doing the same thing didn't get the same result if I was using the cab. So it looks like it's probably the cab.