Could be that because when you bi-amp, the DC path to ground on the mid/tweet section is nearly infinite with a typical crossover. The outlaw amp (and others too) may go unstable during shutdown while the pwr supply caps are discharging and osciallate some as cap voltage goes down, when there is no DC load. Sort of like when you power down an amp, you can still hear audio thru it for 15 secs or so while the caps discharge. Those caps hold alot of energy. Outlaw may not have modelled this condition, or maybe they did and the slight oscillation wasn't large enough to worry about. If the noise isn't too loud, I wouldn't worry about it. If it is loud, I'd probably go back to non-biamping.