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

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)!

My experience with internet ratio is not very positive. Degraded SQ (even with Flac) and the non-interactive mode of music playing are the two areas keep me away from them. I knew with the premium member you could upgrade to hirez music but I still doubt the comparability of the SQ from the streaming service like Qobuz, Tidal, etc. You might want to try the streaming music to sense the differences in SQ.

Secondly, upgrade to a DAC that allows upsampling. There is still a open-ended debate on whether upsampling actually renders better SQ but my limited experience is somewhat positive. You could do this using software (I know you do not want to upgrade the equipment)  I had purposely upsampled 128 kbps AACs to Flac files ten times in size and found the SQ is improved. Not night and day but if you have owned a high-revealing system, the upsampling may bring you surprise.