Cables... what a can of worms...
For speaker cables you don't need much; good connectors, good wire of the proper gauge. I like to use 12 gauge wire with Switchcraft or Neutrik ends. Preferably Switchcraft. Lower gauge is OK, actually, down to maybe 16 gauge because these are very short runs. But if the run was 25ft or more, definitely nothing less than 12 ga. Cable can be just zip cord, Again, I'd use 12 but it's not that big a deal for a short amp run. NEVER use an instrument cable as a speaker cable.
FWIW, when I opened the Seismic adapters, the speaker wire was very oxidized but of what appeared to be proper diameter wire. Definitely not the best cable, but workable. The biggest problem were the crap 1/4" plugs they used. Replace them and you're good. Or buy a good adapter in the first place.
I guess Musician's Gear is OK; I haven't had them apart. I've used them before with no issues. For me, part of the judging of the quality of the cable is the ends they use on them: if they use Switchcraft or Neutrik, they're probably fine.
If you spend a boatload on fancy speaker cables like Monster or other snake oil promising the Second Coming as per your speaker cables, you've wasted your money. Again, good quality, heavy (like 12ga) wire and good quality connectors win the day for speaker connectors. Even 12 or 14 ga zip cord is good; Speakers just want a good reliable connection, not like the front the amp, between the instrument and the amp...
Out in front, the things to consider in the instrument cable is whether or not you have active or passive electronics. Passive electronics are rather susceptible to high end loss due to cable capacitance. Lower capacitance is better, meaning more highs will pass through. It has to do with the way the electronics load the pickups. This has little to no audible effect with active electronics.
All cables have capacitance, but some are lower than others. Interestingly, I've bought various high end cables that 1) were pretty high in capacitance, and 2) were low capacitance but just didn't sound good. FWIW and IMO and to make this short, I've found Fender Custom Shop cables and D'Addario braided cables to be about the best for my money. That's what I buy.
There's a table somewhere on the web that compares capacitance of various brands of cables, if you're interested. And I'm sure there's TB thread or 10 about it. I'd post a link if I could find it myself.
But in short (pun intended) most of the things that you can buy from GC or Sam Ash are fine. It's when you start going to low end companies like Seismic that there can be problems. My rule of thumb: if the price is in the tank, so is the quality.