Tweaks and upgrades that aren't gimmicks

Hard to tell the real deal sometimes, especially when each one of us has different tastes and even most likely hear things differently. When does a new purchase substantially improve the sound consistently over and over for many users?
Let's see if we can come up with a list of things that without any doubt has improved the sound of our system.
This list can include components, cables, room treatment etc.
a better outlet, relatively cheap and should be instantly noticeable and last for years.
a dedicated circuit (20amp). cery cost effective if you can do it yourself. If so, run at least 2..1 for source, 1 for power amp. immediately better...quiter, cleaner, blacker etc.
Pedrillo, man you love lists!
Room treatment cannot really be argued, a "naked" room will sound like crud, everyone has at least some form of dampening be it sofas, chairs, or floor coverings, but going above and beyond this is a huge difference. I have found outlets and dedicated lines to be beneficial and cost effective. I would also add some type of vibration control, doesn't have to be expensive but something like Vibrapods or Isol Pads (I have used both) can make a noticeable and positive change to any system. Cables are a tough one, because the wrong ones can make performance go the wrong way.
Yearly removal of earwax but don't try this yourself as the eardrum is a delicate instrument. Ok, if you are a do it yourselfer approach with extreme caution using a 12 volt drill and a Q-tip.
Room treatments, room treatments, room treatments. Great bang for your buck. Until you do this don't waste money on anything else in your system. This way you'll know what it should actually sound like. The difference can be and often is huge.
More music, whether lp's, cd's, sacd's, whatever, will make any system better.
Room treatments and electrical upgrades. If you spin vinyl, a vacuum powered record cleaning system is a worthwhile addition.
hmmm, this could be quite a list:
1) room treatments
2) dedicated AC outlets
3) vibration control (esp beneath moving components (like CDPs or turntables, but highly effective under ANYTHING with a toroid tranny)
4) power conditioning...depending on the unit.

My biggest sonic improvement came when I experimented with bi-amping.

Splitting the loads between amps on extremely difficult to drive speakers has just made everything move in the right direction. In short, everything is better,

I also bi-amp a very efficient speaker system, but the sonic benefits weren't nearly as great. To be sure there is some improvement, but you would never justify it on a cost benefit anaylsis.

Hope this helps ...

Two relatively inexpensive tweaks that I really have found to work are the Blue Circle Noise Hound III and the Totem Beaks.
The BCNH I tried between source, pre, amp as well up-line. I found the effect the best right before the source, as well as up-line, several feet before the source. Quiter back ground, better resolution. (note, I can not try them down-line, as there are no more receptacles.)
The Totem Beaks, (one set of two, although Totem claims even better results with a set of four) when placed at the front outer corners of the speaker, about 1.5" in from front and side, provided a great improvement to my HT speakers (Athena.) The immediate improvement was focus, less smear, more depth to the sound stage as well as some width to the s. stage. You can actually feel them working, vibrating ever so finely when touched.
I can not comment how they work on my Gershmans, as there is no room to place them on top.
Instead of two stereo speakers in the front, try four identical stereo speakers (two left, and two right),(two in the center, and two outside). Play all four at the same volume so no separate attenuator will be needed. Angle the center speakers outward until the whole wall lights up as one picture of sound. At present your wall probably sounds like it has two "bright" spots with a hole in the center. If you think two speakers give you all of the detail, separation, and focus that you need, you are going to be in for quite a surprise when you try four. There are no gimmicks involved. It's only two channel sound, but it sounds better than SACD using digital decoders. That's my two cents worth.
1. Upgrading speaker crossover network parts. The absolute best upgrade/tweak I've ever heard.
2. Multiple independant & dedicated circuits.
3. AC power/line conditioning.