Both are mixing, so you can set volume for MIC and for LINE independently. Select MIC and set the volume for it; next select LINE and set its volume. So if you have background on LINE, then you can have MIC set at a higher volume. When MIC is active, they will both be there, but the MIC will be prominent. This setup is on page 6 of the attached manual 4/8 ohm amp manual.
The 70V also has a priority muting (ducking) function on the MIC input, so when you activate the MIC input with a signal, it cleanly lowers the LINE input to a preset level while the MIC is active, then brings it back up to the original level when there is no MIC signal. I believe access to setting the ducking level for the LINE input is only via RS-232 control. This is on page 12 of the 70V amp manual. Note that the ducking function is only accessible via RS-232 and NOT the front panel.