Hi Sylvia. Thank you so much for participating in this Q&A. It's quite an unusual stroke of luck in life to be able to reach out to those whom you greatly admire. The soundscape and impact of "Undertow" was a paradigm shift for me as a teenager already obsessed with the nature of recording.
One of the most impressive things about this album is the immense punch in the lowest octaves of the mix while maintaining balance and sustain without clouding up the midrange. How do you approach the low end of a mix? Do you engage in sculpting with EQ or do you specifically mic the bass cab and kick drum in a way that allows for them to coexist from the onset?
Glad you noticed the difference in the low end on Tool's music!
For "Undertow" and "Opiate" a DBX 120X-DS was used on the bass and the kick drum in the mix... mmmmm... maybe just a wee little too much? haha!...
The DBX 120X-DS is an external rack-mount subharmonic synthesizer unit, I've included a photo of one below. It has a very recognizable sound. The way I use it synthesizes one octave below whatever instrument put into it. Here's how I set it up on Tool:
Turn up only the 50hz pot on the front panel, leaving the other three frequency selections turned all the way down. Apparently the DBX was designed for home stereo use, because it has RCA inputs and outputs. Use only the sub RCAs on the outs. I actually only use one side in and out, using it as a mono effect for the kick and bass. I use this DBX unit on almost every mix. If you use one of these be careful, if you don't have good monitoring in your studio you could be turning up those sub frequencies too much. I has happened to me!