Use the 3db rule to balance kick and bass 'first'.
With those two balanced, starting from there the mixdown will be more balanced since all other
sounds and instruments will be balanced around kick+bass...
The 3db rule is a basic rule that says when you duplicate say a kick sound, that sound
combined will add 3db to the overall loudness. Even when two sounds are not the same though, the same rule
seems to apply in general, where if you have a kick and 'add' another sound that is about the same loudness, the RMS value will increase by about 3db...since you've already been gain-staging your faders long before this mixing stage (I hope..), they're already 'about' at a similar loudness...
Beware though that this addition of 3db is an 'RMS' value,
so we need to use a VU meter for this trick, not a peak meter...
mvMeter II is still free and imo one of the best 'free' VU meter available:
https://www.tb-software.com/TBProAudio/mvmeter2.html
To start, download and install it, or use another VU meter, and place it at the beginning of the chain.
Then solo the kickdrum and enable the VU-meter so you see at what level the kick is peaking.
Then change the pre-gain knob on the VU meter until the kickdrum hits at exactly -3db.
Do not change its mixer-fader in your daw though...
Now solo the bass too (add it to the kickdrum) and both now need to be hitting exactly 0db on the VU meter.
So you play with the mixer-fader of the bass in your daw until the VU meter peaks at 0db.
When done, just remove the VU meter from the chain, and your kick and bass are now balanced in such a way that
they form a solid basis for a mixdown.
This works with any kind of music, as long as it has a kick and a bass sound...
For reference, here's a video explaining the same thing :
