Can you please explain the types of problems that could arise from that great a difference of output power. Also the types of problems from using different amp manufacturers.
Assuming passive bi-amping is being referred to:
1)A lot of the power capability of the higher powered amp will be wasted, as I indicated above, since the maximum volume level that can be used will be limited by the clipping point of the lower powered amp. Honest1's point in the post above is correct, and the reduced current demand on the lower powered amp will often (depending on the design of the particular amp) raise its clipping point somewhat, but a large disparity in power capability will still mean that much of the capability of the higher powered amp is wasted.
2)If the amp that has higher gain does not have a volume control, a resistive attenuator (or equivalent means) will have to be provided to match the gains of the amps. That will also often be true even if the amps have similar power ratings, if they are not identical models.
3)If the amps have significantly different sonic characteristics, overall coherence may suffer.
4)With different model amps, the flexibility of being able to vertically biamp is lost. If identical model amps are used, both vertical and horizontal configurations can be tried. The vertical configuration might sound better, perhaps due to reductions in inter-channel crosstalk or other inter-channel effects within the amps, since the two amp channels process identical signals in that configuration.
5)If the output impedances of the two amps differ significantly, as may happen if one is solid state and the other is a tube amp, tonal imbalances might result, depending on the speaker.
The following two issues pertain to any passive biamp configuration, even if the amps are identical:
1)In any passive biamp configuration, if the preamp does not provide separately buffered outputs for each amp you have to be careful that the combined input impedances of the two amps remain much higher than the output impedance of the preamp. Preamps providing two sets of output jacks often have those jacks wired directly together, rather than separately buffering them.
2)If the preamp does not provide separately buffered outputs for each amp, the sum of the capacitances of the interconnect cables to BOTH amps will affect the signals to BOTH amps. If total cable capacitance is too high in relation to preamp output impedance, the upper treble may be slightly rolled off.
However, it is almost impossible for both of these amps to increase in volume in a nearly exact fashion. Even if you find a good match at 10 watts, as volume increases one amp may increase at a different rate.
I don't think that is true. What should be matched is voltage gain, not power output (which will vary with the impedance vs. frequency variations of the speaker). If voltage gain is matched at one output level, and does not remain closely matched at other levels, it would mean that at least one of the amplifiers is significantly non-linear. That is, a plot of output voltage vs. input voltage would not be a straight line, or very close to it. Non-linearity produces distortion, which would be unreasonably high in that situation.