Are There Improvements that Can Be Easily and Quickly Made Without Buying More Stuff?

Depending upon how long all of the speaker & interconnect cables have been in place, I’ve found that removing them & cleaning both the cable terminations & the jacks on the speaker or electronics & replacing them can make a positive difference.I just use rubbing alcohol & cotton swabs on wooden sticks designed for this. Let any residual alcohol evaporate ( which it will pretty quickly) before re- connecting. I know there are specialty products for this but this seems to work just fine. 

Much good advice already. I suggest playing something with a strong bass line. Walk around the room and mark EXACTLY where the bass sounds strongest. Move your sub to that spot. Check for improvement. Repeat with other subs, if any.

Move a bookshelf onto the wall opposite your speakers. This will suppress high frequency reflections, and may make the sound more focused. Keep it that way if you like the effect.

Good luck and enjoy !

Something just ain't sounding right? Pour yourself a nice Single Barrel over rocks, wait 10 mins. 

I agree with all above, but one low hanging fruit is also component placement and wire management. Keep interconnects away from AC cables, keep amps away from sources, try to remove any switching power supplies from sharing the ac circuit. 

It may seem silly, be sure your ear canals are free of wax. I have senior citizen friends who never thought abut this and were astonished when they used an ear cleaning kit from the drugstore. Neither he or his wife had ever thought about it. Happy Listening (once you can actually hear)!