As per title, what was your most unexpected surprise in hifi, you know something that should not work but was a total revelation to your system, whether or not you intended it
Mine was a speaker cable I was given 100 meter drum from a place closing down, they used to install systems in homes and industrial places its got no printing on it or any info but my god its good stuff it beat my old stuff hands down sound wise, it’s not particularly big diameter and just plain copper from what I can make out just thought I’d try it for fun and didn’t expect it to be so good
Also messing around one day very board I had a meter off klotz cable and some very old phono plugs I made them up and they beat a chord cobra set hands down didn’t expect that at all
Sometimes it just happens when it shouldn’t
Laughably a Bedini Clarifier.
Getting a small temporary new main system at 1/8th the cost of my multitude of Naim and PMC boxes (in storage) and finding it did quite a few things better.
The little Luxman/Omega system has me doing some soul searching.
Naims automatic above inflation price rises, wonder what happens this time round!
Power cords and different electrical sockets (UK and US). In my case I knew it would work but wasn’t prepared for the massive changes in sound quality.
My most unexpected surprise was finding how completely different 13 speakers sounded, all costing about the same (of the order of £3k in today’s money. Given that all had had rave reviews in hifi press, and most were top of their manufacturers’ ranges. 10 sounded awful (to me), rejected by the end of the first audition track. One other lasted a little longer, trying to like it. That just left two that were good enough to spend some time listening. Interestingly one I just couldn’t get on with has a few fans on this forum.
How good a 50 quid raspberry pi streamer can sound, and gets me questioning if there is need to spend more.
How IsoAcoustics under the speakers made such an immense difference. How can they? But they do.
Adding NAC A5 speaker cable!
To this day, remember how surprised I was that such a small change could make such a big difference.
Listening to ‘The Dark Knight’ soundtrack at almost 12 o’clock and getting a fright when something started ‘tickling’ my shins…turns out it was the speakers blowing at me during the low bass synth bit!
As mentioned on another thread, setting up my Nova in the living room last week with 40 year old speakers in completely the wrong position and being blown away by the sound.
Muso Qb.
Putting in a dedicated spur for the mains supply. I’d heard it was worthwhile but the reality was still a surprise. Possibly the huge improvement in quality of sound after using an RCM for the first time
Streaming - Properly. Turns out I was just being stubborn.
Finding how good a pair of Linn Kan IVs sounded on my window sill with my Nait 5.
Took me 3 years to find replacements for my ES11s and like yourself I was shocked at just how quickly no became definitely no but also how poor dealers were at hearing the same.
The effect of an Ecosse Big Orange 
( not jokes about 45 please) on my Sky 
box, the difference on sound and vision was the best pound per sound
In the early days of moving up the chain when I had a NAD 3020 amp with matching tuner & CD player and a decent tape deck that were all linked up with the then std el cheapo thin interconnects that came in the box, I decided to make my own harness. It used decent RCA connectors and nothing more than better [thicker] cable, and was all sheathed in external screening linked by a daisy chain -ve wire that went from plug to plug. The cable lengths were as short as could be but long enough to form a nice U shape from component to amp, and formed a one piece harness.
The difference was way more than I expected. In those days I still lived at my parents’ house so didn’t have the flexibility of a proper set up with speakers in optimum positions, but the lift was very noticeable. Up to then I’d believed that better interconnects were a marketing ploy.
A long time ago I went into a shop to hear an Arcam home cinema amp and came away deciding to buy a Cyrus system.
More recently buying sound absorption panels and discovering more bass as a result. I think that previously the bass reflections had been cancelling out the bass.
The sound a Mu-So mk1 produced from a single box.