DIY cables question

I guess my main question is, when wires work, is it black or white, meaning they either connect or they dont, or can bad soldering limit the max performance of the cables or furthermore my entire setup?

Yes. Yes. And Yes.

To make a good solder joint first the parts to be joined must be absolutely clean to shiny bare metal. Then crimped or joined together mechanically to the point the connection is secure and done. Finally, heat with the iron from one side until heat flows through melting the solder which is held on the other side of the joint so that the heat draws the solder through. Use a quality solder such as Cardas Quad Eutetic rosin core solder. 

How bad can a bad solder joint be? I had one channel gradually deteriorate to the point it was intermittently cutting out. This was eventually tracked down to a very expensive professionally built RCA that had a cold solder joint. 

Soldering is just one aspect of DIY, and not even the most important one. Every single bit of the wire affects the sound. If making a professional quality cable was as easy as soldering connectors on the end of wire off a spool rest assured someone would have discovered this and be doing it. The profit margins guarantee it. Forget everything else, this one simple fact is all the proof anyone should need to know DIY cables are a good way to go only for people who enjoy building DIY cables. 

I tried, and had a friend who tried for 30 years. Dismal failure. I know precisely one exception, and Lubos is making $5k cables for $2500, $20k for $10k. 

Unless you are Lubos I highly recommend you take whatever your budget is and research and buy some good used and professionally made cables. Synergistic is my go-to in your price range. My bet is you will be staggered how much better they are.