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

in the "old" days, Pierre at Mapleshade, had you start on the floor, and add a big city phone book, one at a time until "perfect" listening position achieved...was interesting and fun

Are you running your stuff on more than one dedicated circuit?

I was, but I recently read a thread that prompted me to try  ’all-on-one’ and to my surprise I truly think I like it better on one.

There are some things that cost very little that might be worth doing.

Try experimenting with speaker and component isolation - this can be tested for very little cash. I know the debate spikes vs isolators is ongoing, but it is cheap to test.

Get some big cork furniture glides at the hardware store and put them under your speakers. If they help, then better ones will help more and you can plan for the expense. In my case, isolating my speakers was a huge improvement (suspended floor and very heavy speakers). A better alternative is a cork pad meant for it

These are knockoffs of the original version

Vibrapods are inexpensive and surprisingly effective. Get a some and try them, they did nice things for my old DAC, but nothing for my new DAC.

Other than that, I agree with all the others about speaker and seating.

Without spending ANY money?

I'm stumped. Moving yourself or the speakers/subs positions is all I can think about. But I would think you already should have tried that. If money is available then move on to improving your room acoustics. Once you get that right, and only then, move on to equipment and accessories upgrades.

If using Roon and have a fast computer with at least 500 gigs of dedicated space 

HQ player is built into the back end for powerful DSP that can make your digital much much better sounding.