I've tried dozens upon dozens, nearly every passive non-acoustic, non-record player, non-footer tweak, probably 65% of all compact ac plug in tweaks, and several high larger end ac plug-in tweaks, and a bunch of usb, rca, and bnc filters. A headphone filter box. Lots and lots of things, and I've made my own as well.
I always try something with a skeptical mind and let it prove itself to me. I also have two systems to try them on that vary quite a bit, I also try them with headphones (various types) and speakers, with video (UHD, blu-ray, cable, streaming, videogames)- which is often much more of an apparent change than audio, with tube gear as well as solid state, everything I can, dozens of hours of testing for each item.
I've tried to get something from nearly every company that offers tweaks to get a flavor for what they offer, but there's still plenty of things I want to try and I've been surprised a lot when I've thought something would do nothing.
The most powerful I can think of are:
-Akiko Corelli. This is a cleanser tweak. Everything just gets crisper and clearer. This is a really pleasant universal tweak. Some tweaks can have a jarring change, but this one is more of a night and day that sends you into a spring morning where the birds are chirping and there's a gentle breeze.
-Add Powr Sorcer x4. I've tried a lot of tweaks that use a form of qrt technology and this is simply the most powerful. I really like the Telos QNR as well for a more compact version of qrt tech (but one that still requires a standalone power cable), but it's not as powerful.
-Heartsound Audio Breaker Box. This one really surprised me. Works great on the breaker, but also works great with a power conditioner, and various types of them (active or passive). Some tweaks just impart their own specific flavoring to a system, but this one just enhanced what a power conditioner does on its' own. Like say a conditioner enhances the reverb from horns and strings really well, well that aspect is enhanced, and say another one has great separation, well that aspect is enhanced. On the breaker it worked much better than nine Bybee QSE's surrounding it (which was a surprising change on its' own as I was very skeptical that tweaks on a breaker could have an effect that I could see and hear through a power conditioner and other tweaks).
-KAT/Heartsound Audio Eden II plug in. This is a sort of plug-in version of Perfect Path's The Gate/Eden, so you don't need to have an electrician wire it into your breaker box.
-A personally made tweak that cost way too much to make due to the powders I had to order from places like Nanografi and Millipore Sigma and the like as well as other components from Mouser.
-A giant Tweek Geek Dark Matter disk. Tried this on the breaker and in a power conditioner, very potent in both places. It has 1000 grams of the dark matter powder in it.
- A really surprising one was how effective those VPI Magic Bricks are on top of a transformer. I bought one just out of curiosity (as I do with every tweak) and it worked surprisingly well. Not as powerful as the above tweaks, but this was something I didn't have a lot of faith in and it surprised me. Works even on really heavy duty casings like those ECA/Xentek/Topaz/MGE extreme/ultra isolation transformers.
-The strongest compact noise filter plug-in that I've tried is the HiFi Tuning Noise Destroyer; it also easily takes the longest to take full effect, and prior to that it can sometimes be a harsh change in some systems, even a detriment, but you have to give it a bare minimum of 24 hours. I can't guarantee it will work with every system, but when it works it's really strong for something so small.
-One that really surprised me that I got secondhand recently was an Alan Maher Tech Ground Mini. This was surprisingly strong. Makes me really want to get the much larger Maxwell Technical Ground, but I'd need to save up for that. I buy most things via sales or secondhand so though I've owned a lot, I haven't spent as much as it'd seem and I sell what I no longer need.
-Best fuse I've tried is the Alan Maher Fourier fuse. I started out small with fuses as I was skeptical of their changes, so I grabbed an Acme cfc silver cryo as they are cheap and seemed interesting, those are great (and I recommend them for general fuse changes as they are cheap and effective) but have a long break in time and can be a bit harsh before fully broken in, then I tried a Synergistic Blue, QSA, HiFi Tuning silver, and then an Alan Maher Quadcell during one of his sales and I really liked that fuse so I upgraded it to his next top fuse (Ambiance I think), then the Fourier. I haven't tried his newest one, and there are plenty of other fuses I have yet to try (Audio Magic, Telos, HiFi Tuning Diamond, the newer Synergistic top fuses like purple, pink, and master, and some others), but I'm really happy with the Fourier.
-A Paul Pang OXCO usb hub v4, great for my PC interface, and on that front a Corsair AX 1600i power supply for the PC. That thing generates nearly no noise for a switching power supply.
-I can't recall what the best tube is that I've tried on my tube system as I lost the boxes and forgot where I bought them.
-Some tweaks that I've tried have been parts of complete rebuilds so I can't really say how much they change things on their own. This is a case for some capacitors, outlet, inlet, and connector changes, Lessloss Firewalls, wire changes, footers, etc.
