Most of the metal recordings are done with two channels - one mic on the rig to capture the attack and distortion while a direct feed off the bass for the low end. I suggest the Audix D4 and put it right on the speaker. It will capture all the way down to 40hz and has no proximity effect - great reduction of outside sound too. Low tuned bass is low in theory, but what cab are you using? Is it really going below 50hz? You really don't need a heavy amount of low end below 80hz - it's just going to muddy up the recording. Be sure to talk to your recording engineer to use a mic and DI channel and go from there. You can blend both channels in the end and get exactly what you need.
The AKG D112 is made to reproduce the low end all the way down to 30hz (boosts everything between 30hz to 200hz), cuts out the mids (400-900hz), then focuses on the briliance of the tone in the 1khz to 8khz region. This mic can give you the tone you want, but it is going to be very colored in the end. If you want to add in some midrange to the tone, you will have to do it on the DI channel. The tone you produce with this mic will end up being extremely difficult to mix in the end.
Or you could just go 100% digital with Guitar Rig 3 or something. Get as many channels of sound as you want and you can hear the results immediately.
