Internal Wire Gauge?

Agreed with @oldhvymec on the use of solid core. 

Stranded blurs the sound in my experience.  I had a preamp and took Duelund's tinned-copper in oiled cotton, compared one input with the original 16ga wire to another with a single strand of the tinned-copper wire in an oversized teflon tube.  The single strand was superior in every way.  

That said, I personally prefer solid core silver.  I use VH Audio's 99.999% pure silver in cotton for the signal path of all my electronics.  28ga, so I'm even smaller than 'ol mec.  https://www.vhaudio.com/wire.html

When I'm going out to the speakers from the amplifier boards, I use 14ga solid silver from tempo electric (http://www.tempoelectric.com/cables_speaker-cables.htm).  You've got real current at that point, so larger gauge wire is better.

@williewonka Great read, thanks for the detailed post. 

And glad to see we're hearing similarly.  Always good to find another person who hears what you do so that you can pay more attention to their comments and recommendations.

Different wire sounds different in my system and with my ears... even when it's connecting to generic circuit boards.  Cost is low compared to other things in this hobby because you're dealing in inches, not feet.  Easy to try.  YMMV.