I'll share my experience with the first app I installed, a free app called "Equalizer Pro". Hopefully somebody will find this information useful.
You can find the app here:
https://play.google.com/store/apps/details?id=com.soundappsfree.equalizer
First, this is a free app supported by ads, so don't be fooled by the name.
It features a 10-band graphic EQ and about a dozen preset profiles. You can also name and store your own profiles.
I'm presently using it in association with the iHeart Radio app, so it appears it works with streaming apps. I haven't tried it with any other apps yet though, and will update this string if it fails to work with any other app.
With the caveat that I can't speak for the frequencies and decibel levels it displays, the app appears to do what it claims to do. Moving the sliders changes the sound appropriately to the slider being moved. I do notice that it prefers you to click on the slider track rather than dragging the slider, a minor detail. The presets also work as advertised, although I won't vouch for the accuracy of the names associated with the profiles (e.g. if the "jazz" profile is any good for listening to jazz).
Being a software app rather than an actual graphic EQ, you can definitely hear artifacts of the equalizer going about its business. These mainly include slight volume enveloping during music playback, and a change in the overall volume level depending on the EQ setting. The latter can be pretty dramatic, and it's slightly odd to move a slider and hear the volume change along with the tone. Of course, an EQ is a set-and-forget app/device, so the latter issue should cause limited inconvenience.
I'll also note that it's possible that some of the artifacts I'm hearing is the fault of the iHeart Radio app. Again, as I experiment with other sound sources, I'll make notes in this thread.
One bug I've already discovered is that if you switch from the EQ app to another one, then switch back to the EQ app, it crashes. I've reported that.
Note though that these problems are small compared to the problem the EQ solves, namely the dreadful sound quality of the speakers connected to the Minix box. I won't explain that except to say that I neither selected nor paid for the speaker system, and will be upgrading in time. I will however, express my surprise at it, given that the system is branded Altec Lansing, which I thought to be a pretty good brand. Clearly they didn't make their reputation by selling these computer sub-and-satellite speaker systems.
In conclusion, I'd say that an EQ app, at least the one I selected, is no substitute for running the Minix audio output through a proper stereo system with tone controls. I still think the Minix box should have tone controls built into it for the same reason it has a volume control, but until I evolve my setup, this will be the situation, and I will learn to ignore the slight problems created by solving the major one.