Going on the definition of "Tweak" as "a minor adjustment", a couple of things come to mind:
- Speaker *and* listening seat positioning. 'nuff said!
- Wiring my speakers: My speakers are bi-wireable, but I don't bi-wire them. Instead, I take the black/- cable from my amp and connect it to the low-frequency terminal (and then use a jumper to the high-frequency terminal), and connect the red/+ cable to the high-frequency terminal (and use a jumper to the low-frequency terminal). Was pretty surprised to how much effect this had on the sound. Note that it sounds better on some set-ups than others...I don't do this in my primary set-up, but have a secondary set-up where it works really well.
- Footers and weights to help control vibration on my CD player. Had the most success using brass cones and weights. However, eventually I went with a rack that used a design combining constrained layer boards, cast-iron dampers, and other "high tech" items to control vibration and isolate components...after which none of the cones or weights really had much additional effect. Rack was expensive, so I don't consider it a "tweak".
Everything else (eg. power conditioning, room treatment, etc) I have done more than "tweaking" to get the results I want...