Stereo Tool

So auto EQ can do up to 9 frequency bands and bass EQ can do 4.
I get the idea that turning both of them on with overlapping bands is not ideal.
What about splitting them up so that there is effectively 13 auto EQed bands, would that be overkill or actually useful?

Any insight on this is welcome. At this point I know just the basics about frequency equalization methodology. (And the does and do nots.)

After some hours of tuning, I was able to produce some good sounding results. I have Bass EQ working on frequencies up to 650Hz, then I have Auto EQ doing everything above that.
So for the record, using both to complement each other, can be done.

For fun, I also tried to do a two pass version, splitting the frequencies four ways. I never got that one to sound good.

Do whatever works for you. But what we've found out ourselves during the last couple of years, is that a high number of bands (and thus steeper filters) is generally worse than just a few bands. It can work with lots of bands but easily becomes very unnatural. This makes sense: when you listen to music you don't really distinguish 13 different frequency bands. The content in band 11 will sound like very much the same content as in band 12. However when processing them separately band 11 may get kicked down due to some loud sound while band 12 isn't. This leads to "holes" in the spectrum, which sounds very unnatural and annoying.

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Imaging producer & Stereo Tool audio processing consultant

Thank you very much for that answer. I suppose I, like most people, assume that more must be better. I have been finding a number of electronic songs that don't sound good with it. Maybe my next attempt will be less extreme.

This still leaves me with the question of if Bass EQ and Auto EQ were made to be turned on together while on the default settings.

Auto EQ was there first, without Bass EQ. Later Bass EQ was added because it can be convenient to use later in the chain after some other processing was done. In my own presets I don't use Bass EQ, because Auto EQ together with my two multibands already sufficiently control the bass. In the past we've used it to balance deep bass and mid bass compared to each other which can be useful.
It's all a matter of how you design your preset, there's no real "right" or "wrong" way to do it. Stereo Tool offers you a lot of tools to do processing, but what you need highly depends on preset setup and goal. If you feel Bass EQ helps you achieve your goal, use it. If not, then don't.

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Imaging producer & Stereo Tool audio processing consultant

That is the answer I needed.
My mind was imagining up images of Auto EQ trying to do one thing and Bass EQ, when set to the same bands, trying to do some conflicting thing. I feel I know better now their intended uses and how to set them up.

It does the same thing on a technical level but it has a different purpose. Both Auto EQ and Bass EQ balance the spectrum by matching the contents in the bands to a relative threshold that you set. The total audio level remains the same, so highs can only be boosted when something else (bass, mids) is cut. Bass EQ is, well, a second Auto EQ but with a different goal in mind. They don't cause a conflict in what they do except if you set them up like that

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Imaging producer & Stereo Tool audio processing consultant