What was the first power cable that you noticed a difference in the sound?

I started with a bunch of DIY Supra Lorad 2.5 SPC and Belden 19364. The differences they made over stock cords were obvious.

The Supra cleaned up the sound and brought more detail. The Belden energized the music and increased bass quantity, though at the expense of increased noise floor. 

I have since upgraded my cords to assortment of brands that cost average $800-1000 each. There is a marked improvement in sound quality in cords priced around that range compared to those below $500. From that price range to mega bucks ones like Ansuz's and Synergistic's higher ends, negligible. I know, I exhaustively home auditioned plenty of branded cords and where not possible, I did AB comparison of cords in showrooms. Without these higher end cords, my setup had substantially smaller soundstage and less slam. Upgrading fuses in both my integrated and DAC brought also significant improvement in clarity; I use Hi-fi Tuning ones.

These are what I have currently in my setup: 

Mains: Nanotec Power Strada 309 (with Furutech FI-50 NCF)
Amp: SoundString Gen 2 SE High Current
DAC: Divini Audio PT-Z 7N
Sub: Real Cable Citrine
Desktop streamer: Nanotec Power Strada 308 (with Furutech FI-50)
Desktop mains: Synergistic Research Foundation High Current

 

Shunyata NR power cords. Alpha on amp. Delta on Line stage. Venom 10 on SACD player and phono amp. They really do reduce noise so you can hear deeper, wider and taller. 

Run Jim run as fast as you can. Just drop the cords and run. This goes on forever and ever and ever. That’s what they want you to do. It never ends, unfortunately it looks like you got taken for a ride. Don’t feel bad it’s happened to all of us at least once

Have to say that it wasn’t even an expensive cable that made a notable difference in my system. The first one wasn’t exactly an improvement, which was a Pangea MkII. But it still wasn’t one very much more expensive after that one that made a serous improvement, either.

For me, just a well made aftermarket cord with decently fat gauge, quality shielding and that is well terminated. But to that, nothing made as big a difference after some better “cheap” cords than introducing a Chang Lightspeed power bar. Better yet, separating digital front end with a separate power bar from the one used for the preamp and power amp.