You might be surprised how much of a difference EQ can make. As an experiment I once used 12 bands of parametric EQ to adjust the speakers in a cheap, old LCD monitor. Sure, you're not going to get any better bass response than before, but stereo imaging (nonexistent before EQ, perfect phantom localization after), spectral balance, clarity, distortion (due to not exciting resonances in the monitor), etc. were significantly improved. Most people could have added an EQ'd subwoofer to those tinny LCD speakers and been completely satisfied.
One of the many things EQ can't fix, of course, is room reflections, which can be helped by room treatments and speakers with a directivity better suited to the room.
Of course EQ can help with room reflection! In a square room you'll have a resonance at a given frequency, and you can mitigate a bit this problem with an EQ. But usually EQs are used too add bass and do more harm than good.
By room reflections I mean higher frequency reflections that result in comb filtering and spatial and temporal smearing of the sound, rather than lower frequency resonances that result in the standing waves you mention. EQ can reduce the effect of room resonances, but it still can't fix the extended decay time at those frequencies[0].
[0] DRC can improve this, but only within a small sweet spot.
One of the many things EQ can't fix, of course, is room reflections, which can be helped by room treatments and speakers with a directivity better suited to the room.