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. 
I've tried this with Magnepan 3.5Rs, but it requires an active XO. The Behringers and DBXs are cheap, but CRAP. I had 4 blow up before throwing in the towel on the 3.5Rs

I don't see how it could done effectively passively, without crossing out the bass on one and the treble on the other

I discussed with monitor audio and they advised against active cross over and I am not into it either. They did mention that the best results, will be with passive biamp. 
What are the amp specs that needs to be matched (or closely matched). Is it gain, input sensitivity, input impedance, watts per channel.

Gain must be matched precisely.
This Schiit SYS will let you match the levels of the two amps that you are going to use!
Can I ask a question? What do you hope to get that you are not getting now? 
romney80 OP

Yogiboy is correct, just use a $49 Schiit Sys on the louder amp if both amps are not the same gain, so you can make them the same with it.
Then drive the lot with your main preamp.
Here is how to wire it up

Cheers George
The gain of my current amp is 29 db, input sensitivity is 1.4 volts RMS, impedance is 7.5 kohm, damping factor (at 8 ohm) >1000, power output 250 watts at 8 ohms.

I am considering Luxman M-900u, which also has gain of 29 db, input sensitivity of 1.24V/150W (8ohm), input impedance of 34 kohms (balanced), damping factor of 710, power output 150 watts at 8 ohms. 

The gain seems to be matched and input sensitivity seems to be close (even though different watts power at 8 ohms). Will this be acceptable for biamp with my current amp ?.

romney80 OP

Yes close, you may not need the Sys, but get it anyway as it comes with a trial period, so you can send it back, it is only $49

If it’s on the bass amp, then you "can" raise it slightly for those "bass shy albums", or lower it slightly if it’s on the treble amp.

And yes I’m also against active bi-amping as it sterilizes the mids and highs, it’s ok on the bass though and use the speakers passive xover on the mids and highs.

Cheers George
@yogiboy and @Georgehifi. 
Thanks for those suggestions. I would rather not do that, unless that is the only option. But, in the meanwhile, I will dig deep into this. 
The thing to keep in mind, with amps, is that the gain specification measurement is just not that important to makers.

That is, maybe they will specify 29 dB but maybe it’s 28.5 and maybe it’s 29.7.  It's rarely 29.00 dB

Since 99.9999 of their customers will use 1 stereo amp, accuracy here is just not that important. I’m sure it is consistent among the same model but across different models or makers probably not.

So, good to have some way of measuring the relative gain and adjusting if wrong.
let’s say if it is few decibels apart, how much of the impact will this have. In other words, if they are apart, how will the sound quality be affected. Will it be like one amp sounds more than other one ? Or will it affect the coherency in the music ?
A few decibels is actually a LOT!  The difference between 29 db and 32 db means that you would have either too much bass or very weak bass depending on what amp you put on the bottom.

If Erik is correct that gain is not always 100% perfect specification, then the variance might be small.  A .7 decibel variance is not that much, but I wouldn't say it's that bad.  Speakers are never perfect anyway and actually will vary up to 3-5 decibels through the entire frequency range.

If you put a amp on the woofer that is .7 db higher gain, you might have a "slightly thicker" bass midbass.  It's almost like a very slight EQ. 

I would ignore those "1.24V" sensitivity numbers.  It is just a representation of how much input volts are required to have the amp output a certain amount.  The "gain" is the only thing you need to look at.
Sounds good. Thanks for those technical inputs. I am assuming, even if the companies mention 29 db gain, they may vary at best few decimal of decibels. I am OK with that. 
Just one question though. It may be silly. Does it matter even if the amp have different watts of power (like 250 vs 150 at 8 ohms) even if gain is same. 
A few decibels is completely noticeable!! :) You would be changing the sound of the speaker completely. Dude, if you are OK with that, then go with active bi-amping instead. You’ll get more control, more dynamic range and can fix your level mismatches.

Understood, as I said, the two amps which I stated will have same gain of 29 db. So, I will go with it.