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

Great question!

Well, all music is musical pretty much by definition, eh? Or not.

Musical is a totally subjective term. Practically, it means people like what they hear.

Distortion is the key.

Some people like no or low distortion and hear that as more "musical". Low distortion is a proper objective goal, but "musical" is a subjective term and not the same.

Others may find the presence of certain types of harmonic distortion that does not bother the ears more "musical" because it sounds better or right to them. That’s fine....it’s a totally subjective determination.

If you read up on tube amps you will find that they tend to have unique distortion characteristics that many find more "musical", especially with how the human voice and acoustic (as opposed to electronic) instruments sound. Others not so much

On the flip side, electronic amplification and sources are an integral part of a lot of pop(ular) music. The unique distortion characteristics of tube amplification tends to fare less well there, though there are no absolutes. Both tube and SS amps are capable of producing similar very high quality sound for a cost.

Tube versus SS sound is just one common example where the term "musical" often enters the conversation. All it tells you is what each person prefers ie what their own personal opinion is of what sounds good or not.

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The term musical refers primarily to rhythm and pace. As one learns what different terms mean starting as a novice… the real obvious stuff is treble, bass… imaging, microdetails, grain,… etc… one generally learns down a path of more subtle characteristics.

Rhythm and pace is like noice floor. It is profound but not something easy to put your minds eye on. It is the core musical connection… the emotional connection. It is what makes you want to tap your foot or in extreme circumstances get up and dance. The emotional connection to the beat… well rhythm. It seldom happens without midrange bloom… but it can… midrange bloom is the fully nuanced and laid out to hear midrange really helps to close your eyes and get sucked into the music.

It took me decades to actually be able to put my finger on it instantly. I can now, with no problem and gauge how good. Long ago there was a tendency for equipment that had great Rhythm and Pace to lack detail (tubed stuff is much more likely to have great rhythm and pace). This is not true now. Over the last decade Pass amps have gotten much better for instance.

The opposite is dry and analytical. You sit and appreciate all the details and slam, the imaging, but after 45 minutes you get bored. Your body has no desire to move with the music.

I recently auditioned three integrated amps, a Luxman, Pass, and Audio Research ($10K each except the the ARC $6K). The Luxman was high on details… very trebly offset by punchy bass… great transparency… completely devoid of rhythm and pace. The Pass, real sounding treble, nice round bass and a core that made me tap my foot and move in the chair. Audio Research, my eyes instantly closed and I was completely engulfed in the music (not the details and sound… the music… the desire to more and be emotionally pulled by it. That is great rhythm and pace.

I’m going to add to what @mapman has pointed out. Musicality has everything to do with how the amp makes distortion. The main differences we hear between amps, their ’sonic signature’ is in fact their distortion.

To be musical, that distortion has to be benign to the human ear. The only harmonics that qualify in that way are the 2nd and 3rd.

Higher ordered harmonics, the 5th and above, are sensed by the ear and interpreted typically as harshness and brightness. In musical instruments, the higher orders are sculpted by the instrument maker as the tone colors of that instrument. IOW distortion is sensed by the ear in the same way that the ear hears tonality in musical instruments.

Fortunately, if the 2nd and 3rd harmonics are high enough in amplitude compared to succeeding harmonics, the latter can be masked. The result is even though the higher orders are present, the presentation can be smooth and detailed, which is to say ’musical’. Tube amplifiers are very good at this sort of thing, which has kept them going the last 70 years. Solid state has been challenged by this issue because while they typically make less of the higher orders, their higher orders are not masked.

The ear is keenly sensitive to the higher ordered harmonics because it uses them to sense how loud sounds are. The ear has over 120dB range and frankly, a lot of solid state amp designers didn’t take that bit into account, so unmasked higher ordered harmonics will cause the amp to be harsh and bright; i.e. not so musical.

This is a bit of a nutshell description of the issue.

You'll know it when you hear it.