First off, good call on proceeding with the upgrade. You should hear a substantial improvement in sound quality. As a wise man once said; "It might take plastic surgery to get the smile off your face."
The solid vs stranded debate has been around for many decades. It’s a bit like one person saying: "6+6=12" and another responding with: "No, 6+6 CAN’T be 12!! 8+4 is 12!!" BOTH answers are correct.
There are many factors that contribute to how a cable "sounds". Or, as one person suggested: "Cables can’t make a system sound better. They can only make it sound worse." Very good advice, indeed. "Damage control" is the goal. So, doing no harm (or, less harm) is the key to cable design. OEM cables, in general, make things sound worse.
Geometry, choice of materials, dialetric (effect of insulators), shielding, and gauge are a few of the (but not all) elements that determine how good a cable "sounds." If a designer pays close attention to all the above, you’ll probably have a cable worth owning.
I have performed dozens of speaker mods over the years. My "rule of thumb" is to try to use some version of the same cable that the owner selected between the amp the speakers. This cable cable was probably selected by the owner after careful evaluation and the "personality" of the cable is compatible with the "personality" of the listener. IF the cable is available in bulk, and IF the cable can be manhandled inside of a speaker cabinet, then it’s a good choice. Sometimes a "dumbed-down" version of the cable is required (smaller gauge, different material), but even with these compromises it will still be sonically superior than OEM by a wide margin.
That being said, our "go to" cables are solid core. We just have a foundness of a certain manufacturer’s products. Not due to solid core, but they just check all the boxes on our "damage control" list.
On a closing note, short lengths do matter. We were quite curious about this a while back and did testing with various 12" cables cables. And, yes, you can hear a difference in short runs. Even 12" of mediocre cables can do a lot of damage.
It certanly appears that GR Research has done their homework and provided you with a well thought out product. The big question is not whether they use solid vs stranded, but what is the first cut you’re going to pick to evaluate the upgrade?
Enjoy!