Bi Amping... worth it?

Whatta upgrade boom isn't it Dennis?
First place VTL, than considering bi-amping...!
I can only wish the best in your research.

If you will ask speaker manufacturers they will certainly get "upset" since their buit-in crossover is the best match for the speaker and in some way they're right.
By removing the built-in crossover you will loose the value of the speaker and so you should somehow try and listen first if it's true.
Despite this situation I would really recommend you to ask the manufacturer how would you benefit by removing a crossover and using active bi-amplification.

I also plan to bi-amp with McCormack DNA1 on the bottom and VTL MB100 on the top. From what I figured out I can use it either way passive or active: These amps have both high gain(near 26dB), I will have tubie-sweet mids and highs and fast controlled SS bass doubled on 4Ohm load(my speakers never go that low the lowest is arround 6). The triode mode I will only be able to use in active bi-amplification.

So whenever you will look for passive bi-amplification you should match the gain of amps.

If you reed specs with no gain specified on the amp all you should do is to derive an output voltage from the following equation:

Output Power = V*V/R (squared output voltage divided by load resistance)

We have load resistance specified usually 8 or 4 ohms and from there we can derive an output voltage.

Than we take 20log(output voltage/input voltage) and get the voltage db gain.

Now there is a question:

What can be the tolerance between two amplifiers for a successfull biamp?
Please, share.

Patrick, Bridging and Bi-amping are two different issues.
There is horizontal and vertical bi-amping.
In Horizontal bi-amping you use one stereo amp per channel with two pairs of speaker cables/wires connected to the binding posts of a bi-wirable speaker.
NOTE that two channels at the same time from the stereo amp should be fed from one preamp channel so it figures you'll need at least a Y-connector.
In Vertical bi-amping you use each channel of each stereo amp for each pair of speaker binding posts.
NOTE that each of the right channels of your stereo amps should be connected to the right channel output of your preamp and so identical for the left channel so there you'll also need at least a Y-connector.
As it was said above for passive bi-amping it's mandatory to have amps with the same gain or simply to use the same brands of amps.
If you want to biamp 500W/ch amp with 20W/ch you must have an active crossover since the 500W/ch amp will most-likely have a voltage gain of 40dB and 20W/ch will have a voltage gain of 20dB. You will not need to use Y-connectors if you will use an active crossover. The built-in speaker crossovers must be removed prior. The approximate crossover freequency with 500/20 combination is 6kHz i.e your 20W/ch amp will work with freequencies above 6kHz.

Now about benefits biamping either way horizontal or vertical using two like yours existing amps 500W/ch it depends on the speaker if it's voice coils can take 1000W/ch and not to get fried. Also I would recommend getting a dedicated line or make sure that your wires can hold up upto 25 Amperes before you play with this mega-power system. It might benefit for sure if the above conditions are met.
If you're about to use 20W/ch tube monos you should choose an active crossover freequency approximately at 6khz meaning that your 20W/ch monos will only see freequencies 6kHz or above and your 500W/ch Spectron the rest of freequency spectrum. In this case you benefit by limiting the "visible" spectrum of each amp and let them be more resolving and linear on their new "visible" freequencies limited by low-pass and high-pass filter of your active crossover.

Drrdiamond,
Is there Krell amp 20W/ch??...

In all logical sence in order 20W/ch to have the gain arround 30dB the input sencitivity must be 8 milivolts i.e. something between the phono and preamp. So theoretically in such amp you could plug in high-output cartridge directly. Or if you want the challenge please find two amps 500W/ch and 20W/ch with the same gain... I'm already intrigued...

now to finalize...

It's all logically obvious that in active biamping we limit the "visible" freequency spectrum of each amplifier thus easing the task of amplification. It's beneficial for ALL amps in any combination BUT it also changes the speaker curve that might not be beneficial and needs to be checked and researched before the final setup.

There is no way in real life to connect passively 20W/ch amp with 500W/ch amp since YES, the output power does something to do with the amplifier's gain since it's a ratio between input voltage and output voltage.