First, let me say I really appreciate your feedback. I was very much hoping for these kind of comments.
Especially your "Big variations are less welcome" was valuable to me, not because I wouldn't have known about this at all, but (like almost everywhere) it's about understanding how one's own preferences relate to those of others.
I also think it's important to consider, that players might hear the music quite frequently. So, while not unimportant, the first impression might not always last. I did change my opinion about some of the music tracks in STK quite considerably over time…
I am a bit surprised you'd choose the music of XR591 as an example, because it kind of violates almost all the general rules in STK's
Music guidelines.
For the most part, focus on melody and harmonic progression, rather than making music that is purely rhythm-based or is too ambient.
A good length is usually from 60 to 90 seconds
XR591 music focuses on a very strong rhythm and bass line. The first (kind of) melody doesn't appear until 0:53. And considering, that the overall length should be between 60 and 90 seconds, that's very late for a melody to appear.
Don't get me wrong. That doesn't make it bad music, exactly the opposite: It's easy to realize that this piece does what it wants to do very effectively. However, it is very repetitive (on purpose). So to me it doesn't match the criteria of STK and those you gave.
Finally, I do have to confess that I changed it locally (it's just not my music but also) because it actually did distract me a lot while playing...