Please keep in mind your budget and the quality of the amps. In general a $2000 stereo amp would most likely be better than 2 $1000 stereo amps used to biamp or 2 $500 monos.
Having said that the best options in order are 4 monos to biamp, 2 stereo amps to biamp, 2 mono amps and lastly 1 stereo amp.
Having said that the best options in order are 4 monos to biamp, 2 stereo amps to biamp, 2 mono amps and lastly 1 stereo amp.