What is meant exactly by the description 'more musical'?

I guess it means different things to different listeners. To me it means the amp and/or system it is in produces a sound that is closer to the sound and, more importantly, the soul of real music than to the sound of hi-fi. This may also be attributed to whether the said component has a top-down or bottom-up presentation. Just remember that the music is in the midrange.

I would add "continuousness and coherence" in the sense that an unmusical system can make it sound like the music is being played by musicians who don't quite gel together and can't play "in the pocket".

That's not an exclusive description, however, since I also find etched sounding systems to be unmusical i.e. systems where the components emphasise the leading edge of notes but are harmonically lean or thin. Such systems can be coherent, but tonally, they don't sound real.

Mapman's on it.  As I worked through my hobby system that uses a recapped pro audio Crown PS200 to drive a pair ot mellow big baffle Wharfdale Linton 85th Anniversary Heritage speakers and small REL sub (all connected a speaker level with Mogami 3103 cable), the more I reduced distortion on my input chain by improving the DAC, upgrading the interconnect cabling and fiddling with the gain on an unpowered Schiit Sys preamp....the more "musical" the output became!

Those other concepts like imaging and sound stage become more real as the signal cleaned up. Was pretty amazing how much more enjoyable it was without changing the amp or the speakers.

This is not a hard question 

Simply put musical more musical means 

“more real or realistic”

Good luck Willy-T

For me "more musical" it means you hear all the instruments in more clarity and detail.