Bi Amping... worth it?

Yes I was referring to passive bi-amping. You will need same amplifiers or matched output gain as you suggested you have. With active bi-amping (more expensive) you will need an additional crossover but this will allow use of mismatched amplifiers. Based on the scenario you outlined above I would guess passive would work well

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.

If you have matched amps and are passively bi-amping, is it better to run each amp to one speaker or 1 doing LF and 1 doing HF? Is one of these set-ups called "bridged" and the other "mono blocks"? If so, which is which? I'm embarrassed to ask, but I have to start somewhere.
Also, I have a 500w per channel Spectron Musician II amp. One of its many qualities is that it just sounds like it has oodles of headroom. Does the sound benefit as much from doubling up high powered amps? Or is the improvement curve much greater when bi-amping with stuff like 20W tube amps?
Thank you for any assistance in furthering my education.
Cheers,
Patrick

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.

The "power" of the amp has nothing to do with the gain.
For instance, ALL Krell amps have virtually the same gain, regardless of the power rating.

Richard