Passive bi-amp question

I understand this might have been discussed in the past, but wanted to get some better insight as to doing it correctly. I am looking to biamp (passive only, not crazy about active biamp) my speakers. I have MA PL500II, and after discussing with monitor audio, they strongly recommend passive biamping. Trust me, these speakers are already amazing with just stereo amp driving them now. I am trying to push the envelope to see if I can get best out of it. 
I am looking to use different amp brands for biamp. I know, ideal will be to use same amp or at least same company amp, but wanted to see if anybody have done passive biamp with different brand and what was their experience, pros, cons and how did you guys made decision while selecting different amps.
What are the amp specs that needs to be matched (or closely matched). Is it gain, input sensitivity, input impedance, watts per channel. Or in other words will results be still good if the amps have different gain, impedance, sensitivity and wattage. Anything else should I be looking for. Any valuable thoughts will be appreciated. I will be using same power cords and speaker cables in both (including the lengths). 
Currently I have thrax dionysos preamp with audia flight strumento no.4 mk2.