Summarizing the sound (details below): The Focal Spirit Pro not only sounds very good, it sounds right. I came to this conclusion while listening for colorations of various kinds and finding none, but especially when hearing bass lines that seemed to render more perfectly than with the other headphones I have. Many of the music tracks listed below sound as though their final mixes could have been monitored with this headphone.
When I first put the Spirit Pro on, I thought it sounded rich and dark, but not as rich as the Pandora VI for example, which has an enhanced signature at both ends of the spectrum. What I came to realize is that the Spirit Pro has a 'soft' high end, similar to the Shure 1540 and Beyer T51p, and noting that headphone soundstage and other properties are critically dependent on having a full treble response, I compensated for that partially with a slight increase around 3 and 7 khz. My adjustments are not based on my hearing, but on comparing the sound to the other headphones noted above, to try to be as objective as possible. In any case, whether you play the Spirit Pro flat or tweak it as I did, the overall sound is excellent - from a tight and solid low bass through an uncolored midrange to a smooth and detailed treble that's free of the sonic irritations that plague many headphones, even some of the more pricy variety.
Although I describe the Spirit Pro as somewhat warm and dark sounding, it's not bassy nor is the bass anything but tight and detailed. This is probably very close to a true-neutral bass, but where many of the so-called neutral headphones have a deep bass rolloff, the Spirit Pro holds up well in that regard. The music samples below have comments about the deep bass that I hear and feel with this headphone, but even those comments don't convey the feeling of hearing great bass lines in tracks I'm familiar with, but haven't enjoyed as much as with the Spirit Pro.