I just got 5 of Alan Maher's CBF's (circuit breaker filter) and I consider them the best bang for the buck tweak as well as the most immediately noticeable without a doubt as well.
They are a $25 passive piezo electric filter you put but inside and outside of your breaker panel. They are 2" X 1" X less than an inch tall in a small p-box. I put one on each zero gauge wire feeding my panel as per his site and on atop the breaker that feeds the outlet to which my system is connected.
Alan recommends the one for the breaker be put outside the panel with double face tape but the outlet that feeds my system is the top left breaker I could simply lay the CBF atop of it allow it to have more surface area contact.
The last two went on the 20A IEC's that feed my LF amp and CDP. All units were wire tied to their connection point except the one that feeds my outlet. For that one I put a small bed of silicone and simply laid it atop that. The breaker panel cover serves to keep it from moving as well.
Well listening after that 30 minute install was really freaky. My first reaction was wow, my system doesn't sound like this. This is like a total upgrade. I do a lot of tweaking and some are good, some great and some make no difference to my ear. I never have a stake or hope for the outcome I simply listen and decide if I will keep it. These are keepers and the easiest quickest decision I ever made on a tweak.
Other tweaks for AC I do are: PS Audio P-300 (sources only) Richard Gray choke and Al Sekelas R/C networks ( DIY on AA and my website) Oyaide outlet AND WPC-Z outlet cover and upgraded power cords.
The first track I used was the title track from Steely Dan's "Two Against Nature". It sounded so much better in many ways. The back up vocal were more present intelligible and took a more pinpoint place in my HUGE SDA sound stage. I heard the baritone sax "blurts" (@ -4:19 and -2:40) in the right channel I never heard before. The clap machine they use was much less annoying and the bass had a new power and authority. No real frequency extension just more power and dynamics. The listening chair was vibrating more and in new places. So was I!
I used several other reference tracks, ones I use each time I change something in my system, the result was the same : Wow!
So if you have a few bucks to spare try these, you can return them for a small restocking fee if you don't like them. All those years I never did anything to treat my AC saying to myself all my components change AC to DC inside and filter it, why do I need to fool with my AC delivery? Well most of the tweaks I have done in the last ten years have been to my AC and they have made a huge difference and increased the size and quality of my SDA image and the fidelity too.