NO! I use solid core a LOT on my stuff. Some say stranded only, others could care less, because no one ever looked.. There is a lot of that in commercial speakers.. VERY bad wire securement, (floppin in the wind) small cheap wire. A lot of wire crimps that are just bad, You have to use compression/dimple pliers, solder terminals or solder direct. The cheese ball flat crimper from cheese ball express, NO.
The crimper I have used for 45 years have ORANGE and BLACK handles. Simple! Every mechanic/electrician, knows what I'm talking about...
The cables to your speaker, well you go as crazy as you want. I don't..
Inside, look before you leap, ay!, Just looking at the BAD things and cleaning them up. That alone might surprise you, with good crimps, wire securements, and binding post...Copper for the bass section, silver for the mids and no clad anywhere. You'll get really surprised..
Now don't tear stuff up. One thing at a time, above all. HAVE FUN...
Regards