I was off a lil - this is how mine looks in my preferences.cfg:

[log]
to = logs/system.log.txt
level = * error
If you don't have a [log] section, just add one at the beginning of the cfg file.

IIRC you just put "debug" where "error" currently is in mine, and it makes the game spit out a lot more info when it generates the log. I'm having trouble finding it via search though to confirm it - I'm pretty sure I saw it on here though. Prolly some obscure post...