I have a tone knob on my Microcube instead of an EQ. I've been told that it adds or cuts treble, and doesn't affect the mids and bass. True? What about my tone pots on my Agile Al2000?
A tone knob like the one on your guitar is just a high-pass filter. It only lets frequencies up to a certain frequency through, and you manage the cut-off point by turning the knob. As you turn down to 0, more high frequencies (treble) are sent to ground instead of going through the rest of the guitar's circuitry and out the jack to your amp. So your Microcube's Tone control will do much the same as that.
Electrical Engineering student here!

Blue strat is right, it is a high pass filter.
If you want to have a true eq system, then buy an eq pedal or get an EQ rack mount system. However with merely a microcube, i think it would be overkill.
Depends there are many designs of tone knob. On the guitar its simply a filter that only lets highs thru. On an amp or pedal many times the tone will let more bass thru turned one way or more treble thru turned the other way. Doesnt do much if anything for the mids. So to get more treble you sacrifice bass or viceversa. Its a simpler and cheaper tone control than a bass, mid, treble set up.
As tackleberry says, it depends on the tone knob in question.
Sometimes, it is wired as a bandpass filter, which will cut high AND low at various points, so it will effect everything.