This thread has given me a lot of food for thought, too, and has sharpened in my mind the real reason I prefer flats for many types of music, both to play and to listen to (for example, my tastes in more 'organic music' run to Brasilian and African , reggae, acoustic jazz, where on the other hand I look for something else as well in rock- the excitement of the high volume, the pure sensation of it coming from it's own new sounds and musical language)
First off, I'm not sure I agree with the premise that rounds sound like a low tuned guitar, and that flats give a more solid bottom, etc. That can be true, but it depends more on EQ, amplification, and how you use them than any great fundamental difference in the strings.
Live, at least, I can get a great big supporting groove going with either, for example, and though different strings can make me play differently, in the end it's up to my choices how I'm going to sound; the string doesn't dictate it. I do prefer in general letting an instrument speak and i use fairly flat EQ, but both types can be used to cover many styles and tones, and do overlap a lot.
Anyway, to my main point.
I have a theory that we respond to the human voice very strongly, and that most melodic and many percussion instruments through different cultures and musical styles imitate the human voice to a greater or lesser degree. It is natural they do so, for many reasons obvious to me, that I won't elaborate here.
The bassists I most admire usually have a very vocal style of note shaping and tones that can sound very human, and voice like. I find it much easier to imitate these sounds with the natural attack and decay of flats, and L + RH, or foam muting can accentuate this even more, not necessarily the same effect as when using rounds. 'Modern' bass tones with their wide frequencies, infinite sustain, etc, owe as much to the advent of the synth, and our ears becoming accustomed to and accepting of less 'natural sounds' than ever before. I'm in no way making a value judgment about whether this is good or bad, it just reflects the way we live and experience things now.
But even the hugely artificial guitar and bass sounds we use now, in the hands of the great artists, tend to end up sounding like variations of human expression, from the wail of a distorted electric guitar, to the way gated and wah-ed synths sound.
The same can be said for many of the arts- where modern art is self referencing and uses modern symbols or shapes and colours to describe the human condition, before it used naturalistic portrayals of nature and humans to make it's point, or made a point simply of that. Same for poetry, and literature, to a degree- what meaning does a poem or story have to us now that uses animals, plants, and seasons to illustrate its meanings, for example, when we have little or no real life knowledge of the same?
So while not denigrating the exciting and fully modern capabilities of a roundwound wearing bass, my preference for the sounds one can get from flats is due to it being in many ways a more 'natural' form of expression, a combination of a drum and a human voice, in a matter of speaking. They can sound more like gut than metal, too, part of the whole musical vibe that stretches back to our ancestors. That is the big difference to my ears and for my purposes.
I hope this hasn't been too obtuse or hard to follow- I have given few examples and have made too many very broad generalizations, but it is a very important part of one of the ways I play bass, and it guides my instrument, string, and amplification choices for some of the music I play, too.