fuses - the $39 ones or the 85 cent ones

Good, A properly treated room should be done 1st.
If you have a properly treated room, than on to the next project. My personal space, in a small house, has 8 walls, 2 at 45's, and an asymetric vault ceiling. Either an amazing opportunity or a nightmare. About the only major positive is that I don't have any real bass 'hot spots'. When I first put a system in this room I had an echo from the far, short wall.
Since I was in debt up to my nostrils, my treatment of choice was a small woolen tapestry hung about 2" from the wall. That helped imaging etc etc no end.
My next project, nearly 2 decades later is the purchase of some OC703 to treat 1st reflection points (1 wall at 45 is about 11' tall, maybe 12' from RH speaker....)and between the speakers. A local hi-end store demo'd w/ and w/o such treatment and it is clear Maggies benefit enormously from this...the backwave from them makes for some unique problems.
My point in dragging DBT back into this is that the PLACEBO effect is alive and well and living in everyone. Does my car feel better after an oil change / wash job? Yep! Is anything really different? Nope

All you gotta do is leave the room while a trusted friend does or doesn't change the fuses.....Do this 10 times over a period of 5 or so weeks and see if you can tell more than a chance 50:50 A score of 90 is definitive and warrants a good crow eating. 60% is still within chance. For validity, you really need more like 20 trials, but for the sake of arguement, 10'l do.

I'll also check out the system pics.....Without even looking I KNOW there are a bunch of good room treatment ideas, stuff I never saw or thought of. Maybe some of them will increase the WAF, 'cause I see problems with room treatments!
I'll also admit to some jealousy, in advance. I simply don't have the space or money to make a dedicated HT / or listening area. Everyone that simply isn't near-filthy rich has to compromise. I know most everyone here has done so.

I have not read this whole thread so maybe some of this is redundant.

I would say the results vary quite a bit.
Obviously it also depends where in the circuit path the fuse lies as to how it will effect sonics (as well as the particular circuit as well) and the resolution of the system.

In my PS Audio Solid state control amp - the Hi FI fuses provided a nice improvement that was subtle but noticeable.

On my ATma amps the fuses (one of which is in the signal path to protect from tube blow outs) were a large improvement in inner detail among other aspects- and was not at all subtle. I have suggested these to other owners , one who thought it was a much bigger upgrade than going to better power cords.

I put them in my sub's amps and I can't really tell if there was an improvement (though I didn't then pull them out and listen again- so who knows.)

These companies should create a user database for people to fill in what component they upgraded and what the improvement was. I think it would help people to feel confident they were getting their money's worth.

But to me the cost is small enough that if it makes 50% of my components better - it's money well spent. This goes against the law of diminishing returns we often speak of. If $80 in fuses makes my 14K$ amps even 10% better, it's worth the trouble.

Wow! 14 large for amps is beyond my entire investment, at new/retail. Even counting my non-audio stuff like TV and DVD player (OPPO '981).
As far as I can tell, given that I have a non-optimum room, I could spend 3x on equipment/gear and not get much or any better sound.
Now, If I spent some loot improving the listening space, I'm sure that there is probably a better equipment set.
Everyones value/cost curve is different. But for now, I think I will get more out of a better space for say....300$, counting a couple boxes of OC703, some wood / coverings and mounting hardware than I could out of half that in fuses....
2 for the amp / another 2 for the speakers.
At that point I may be willing to go nuts and ship off for some standard ceramic fuses. If they make ANY difference I'll be surprised enough to ask for some fuses for Christmas!
Now, I dont' believe I mischaracterized anyones statements.
That there are believers and skeptics is a given. In my last post I used DBT as an example of how the Placebo effect plays into peoples wishes / wants / desires. I, better than most realize the difficulties (practical impossibility in Hi-Fi) of organizing such a test. Being on the skeptic side makes me the nut! If anyone would perform some due diligence and just have some healthy skepticism....
I will remain open to the tuning fuse.....But would wish for someone to be able to reliably tell the difference...fuse in or out, with some degree of confidence. This is the 2nd or 3rd time I've made such a request. No Takers? If I had these fuses, you can be sure that I'd try it.

Don't mention double blind tests as they are invalid tests for audio.

Tbg, wow, where did that come from? To paraphrase you, certainly you saying DBTs are invalid don't make it so. As Magfan asserts, conducting DBTs would be difficult with hifi but not impossible. Of the actual DBTs I've read about only ten percent or so have shown people hear a difference between the items under test. Those people who statistically show the ability to discern differences are usually practiced listeners. Which you probably are. But again the items under test were items that are accepted, even by most skeptics to have a difference.
IMO many of those people in the audio world who are dead set against DBT have something to lose. For instance, if I sold an item such as a battery operated clock and claimed that by placing the clock in your home your audio system would sound better I wouldn't want that subjected to a DBT.