Kick and bass: judge them together
A low end where the kick and bass sit as one, with the weight in a single tuned place and every note even.
The problem
The kick sounds great in solo. So does the bass. Play them together and the low end turns into a blur, and the obvious fix is an EQ cut on one of them.
What to do
Loop a passage with the kick pattern and the lowest bass notes in it. Listen to the two together from the start, and don't solo either one again until you've decided what each is for.
Settle the level first, then the note lengths. A bass note that rings across three kick hits will fight all three, whatever its tone. Shortening the note in the MIDI, or with a fade on the audio, often fixes more than any EQ move.
Then decide whose weight sits where. Whisper of Satan puts the weight in one tuned place and pulls down what sits below it. Choose the range with the Center 40 Hz and Center 100 Hz switches, set a small Amplitude, then sweep Frequency while the whole line plays. Put it on the bass and tune it away from the kick's main note, so each has its own spot.
Change one thing at a time. Moving level, EQ and dynamics together makes it hard to tell which move helped.
When to stop
Stop when every bass note sounds as loud as its neighbours against the kick. If one note booms, the shape is tuned too close to it. Small speakers tell you whether the notes read. They can't tell you how much deep bass is there, so check on full-range speakers before you call it done.
The test
Play the passage on a phone, then on your monitors at the same level. On the phone, every bass note should be audible. On the monitors, the kick should land without the bass dipping around it. Then play a different section, because a setting tuned on the verse can miss on the chorus.
Rory