If you're mid section is powering the entire passive set then the mid-hi setting should be something like 20k (as high as it will go). If you set it lower, you are basically cutting out all of your high frequencies.
The mid-hi/mid-low 3 way setting is designed to power a midwoofer in a bandpass configuration when used in an active set-up... It's not designed to power a passive network at basically full-range, altough it will probabaly work.
I would try to raise your mid-low crossover and lower the sub, as well. I imagine at 100hz, this set-up is pretty boomy... but it's all a matter of personal preference.
FWIW, these are my current crossover settings, but bare in mind I am running active...
sub 50hz @ 18db (polarity reversed)
mid-low 125hz @ 6db
mid-hi 2k @ 6db
hi 4k @ 12db (polarity reversed)
Bare in mind that these settings are very much vehicle dependant and are also much a matter of personal preference.
