12 gauge zip vs Cat7 DIY speaker cable - oh my

I've got a NAD C3356 driving a pair of Focal 926s and a Gallo TR2 sub, with material coming from iTunes through optical into Cambridge Audio DAC on the 740c deck. Speakers connected via 12 gauze Radio Shack zip wire. OK sounds good. I got curious about what would be better within a budget of $600 or so (I need 2 sets of cables, one to sub, the other to the speakers themselves). I explored a couple of 'home trial' offers and got turned down. I was open and mentioned my research and latent skepticism and I guess that did it.

Curiosity still ate at me, so I bought 2 100 ft Cat 7 ethernet cables and assorted terminations and took a shot at my own set. I was disappointed to note that the 26 gauge wires were stranded, not solid, but I decided to press on anyway. I used three lengths for each cable (pos or neg). I have only cabled up from the sub to the speakers. It's quite tedious making these.

I left the zip in place for reconnection, installed the newly created cables, and listened to the stock set of tracks I always use to test changes. I've got a listening log for these tracks which are a playlist, and I make a fresh set of notes for each cycle.

I can't say I'm skeptical any more. I honestly really didn't want it to be that much better, because now I'm thinking I might as well pop for some commercial cables like Transparent Wave or their next step up. Mine don't have fancy oxygen free terminations, etc. I bought the spades at Home Depot to be honest and crimped them. I went back and forth between the zip and the new cables 4 times, which was tedious in and of itself. The notes were consistent each time - it was like a light bulb going on and off. There was no gray.

The single greatest difference which I never thought I'd hear was the added echo or decay in certain instruments and vocals. A classic was from Sting's "Desert Rose". That final 'ding' tone in the final passages held for seconds longer before it disappeared when the new cables were on. Natalie Merchant's voice in "San Andreas Fault" had far more resonance and a more forward presence with the new cables, and across tracks cymbals in particular had a brassy sound (I don't know how else to put it), rather than a simple hiss. You could almost see the cymbal shaking from the sound.

It's spooky. I'm done with the serious listening and am typing this, but that playlist is still playing. I keep looking over my shoulder as I'm typing when the music is seriously different than it was on the zip wire. Right now "Halo" from Depeche Mode is playing, and I heard the membrane of the big kick drum in the first seconds of the cut, and I'm hearing the lead vocal's breath intake along with distinctly more echo. With the zip this just isn't there. It sounds all right, until the other cable goes on then you hear something different.

I never would have believed it. I'm not shilling any cables or saying everyone needs to use this or that stuff. This is just one middling setup in a room with horrible acoustics and the A/B test (can't be blind since I'm swapping the cables) just can't be ignored. I frankly didn't want it to be better let along this much better because I really don't want to spend the money (just found out I owe Uncle Sam ~$4K) but now I see what 'might' happen if I do. I now have to ask myself if I can be satisfied with a very amateurish cable set that cost $100 or if I want to spend 6X that for the 'real deal'.
what length are you using for your main L/R?
2 meters from sub to either speaker. The three Cat 7 cables aggregate to just a bit over 12 gauge equivalent if the charts are to be believed (24 count 26 gauge wires per pole). The NAD's nominal output is 70 watts I believe, with some headroom. I'd expect 100 watts for peaks.

Amp to sub is 3.5 meters, 12 gauge Radio Shack zip wire.
Nice one. I made some speaker wire using solid core cat5e and love the sound. Very transparent. The trick with stranding is to use a drill and a vice which saves hours of work: Take two individual 24awg strands and use the drill and vice to wind them in one direction, repeat with another two strands in the same direction, then wind the whole thing in the opposite direction. If you have four ohm speakers you'll need to use three strands in the first part of the process. Terminate with bfa bananas or spades and some heat shrink. Total cost $50 and they sound amazing.
I'm not sure I'd spring for the more expensive cables if I was in your position, the difference over the diy ones will be marginal until you upgrade your amplification.