When bi-amping, the only way to adjust the levels of the two signals is externally from the receiver or processor (I'm not aware of any units that include internal level controls that would allow this). That's typically done with gain controls at the amps (controls that are not as common on multichannel amps as on older stereo amps), unless you actively bi-amp (in which case the external crossover network could provide some gain control as well). I'm going to hunt around a bit for an old thread that discussed this very issue, as it included some discussions of the dangers of separately adjusting treble/mid and low frequency levels. Basically, though, the speaker designer based design decisions for the drivers and the crossover network on the inputs getting equal magnitude signals, so altering those levels can have repercussions on the overall frequency response of the speaker. I'll see if I can find that thread.