If you click on the "?" at the top right corner of any PD tool, it will bring up the help file for that window.

Here's the one for AudioEditor, although it doesn't explicitly mention the Custom preset. That's actually the last entry on the Preset dropdown list, and the tool will also automatically switch to that if you make any changes to one of the existing presets.

However, the tool doesn't seem to remember the settings if you change to another preset, or if you edit the modifed clip in AudioEditor again. I'm not sure if that's a bug or if it isn't ever meant to retain the custom settings, but it would sure be nice if it did.

The normal way to transfer existing edits on a clip to a new one is to right-click and choose Copy keyframe attributes then right-click on the new clip and choose Paste keyframe attributes.

However, the AudioEditor edits aren't transferred, not even if the audio tracks are unlinked from the video sections first. You may want to make a screenshot of the equalizer so you can match the equilizer controls on another clip.

In older versions of PD that used WaveEditor, the custom settings are retained so this does seem like a bug. Unless someone else posts of a different outcome or workflow, I'll report this issue.