How to change equalizer presets with Tasker?
If you use Apex or Nova launcher: they've an integrated "activity browser". Just long-press on an empty place of your homescreen, select to add an activity.


Activity selector in Apex Launcher (click image for full size variant)

Browse to the sound app, and see what activities it offers. As those "preset selectors" must be available as "screen" to interact with the user, they should have declared an activity; with some luck they did so "publically", and then its listed here.

As you can see in above screenshot, there's not only the "descriptive name" displayed, but along with it the package name (if it's the app entry, as the first one in the screenshot: "LocMgrPD" = "com.android.logmgrpd") or the activity to call if it's an activity (second and third entries, e.g.).

Once you found the matching activity, you can use that to call it from Tasker.