Cheap tweaks...What would YOU reccomend?

10 Vibrapods of #5 under each Speaker.(200lb speakers) .
The speakers may wobble for a few days but will settle in . It really makes the tonal quality of the bass notes clearer.

Audioprism Ground Control for speakers

Are you using a Terminator on a digital OUTPUT?

Just tried the Bybee Quantum Signal Enhancer they retail for $119 each. This is probably the lowest price ever for any Bybee product.
And you know what?.. It works great on Video and Audio!
I even put one in my breaker box over my dedicated room breakers.

Sabai, With Audio, there is a much darker, deeper soundstage, additionally, there is much more information on the outside of the speakers.
instruments and vocals sound distinct and are very separate.

With Video, the colors really pop, and the sharpness is excellent on my 4K 85" Samsung.

Sabai, I liked them so much that I bought 11 of them.

One is placed over the breakers. One each under my 2 DIY power strips.
I have one each under each of my Audio components. I used some Cardas blocks to get the Bybees up as close as possible to the underside of the component.

On the Video, I have one each under my cable modem, cable box, Blu Ray, and attached one to my TV.

Surprisingly there is a break in time, which makes it a little hard to decide on final placement.
The Video improvement was instantaneous, but the Audio took a few days to open up.

Justubes2, The Bybees are different than the others. Not sure what formula of crystals is being used but it goes beyond the other items you mentioned. My first impression on audio was not good. The sound was muted or just with no treble. Kind of like when I tried those RF clamps around my cables.
But, the sound did get better and better as the days went one.

11 of these is probably overkill, but that's the way I am. Besides, about half are used on my home theater equipment.

I'm getting ready to post another "next best tweak" item that costs about $9 each.