Aggielaw - An amp doesn't "think" about what it sees. It only sees what it's hooked up to and that is it. Each channel is on its own and has no idea what the other is doing (this is not absolutely true because of capacitive coupling but in the grand scheme, the channels are independant). As a result, there cannot be an imaging issue.
However, there is a current imbalance in the amplifier between channels since one side sees a (generally) easier load in the treble and the other has a heavy load for the bass. BUT, this issue is also moot because a good quality amplifier will have a power supply of sufficiently low output impedance that the imbalance will not be reflected in the crosstalk, nor as a power rail voltage differential.
The advantage is that the power supply is much less taxed than it would be driving the entire speaker. The reason for this is two-fold. One, the high side is generally a 3x easier current load than the bass.
The other is that you no longer have the impedance of the highs and lows in parallel. Using a single pair of cables on a biwire speaker means that the amp sees the speaker's impedance for the highs and lows in parallel. Let's say the bass is a constant 4 Ohms and the highs are a constant 10 Ohm load. The sum total of both together is 2.8 Ohms! The reason this isn't reflected in the impedance plot is because the plot is generated by a sweep, so only one frequency is produced at one time. But with music, you have thousands of frequencies all at the same time which effectively puts the two halves of the crossover in parallel, as far as the amp is concerned. This is a big problem for an amplifier power supply and is also the reason that some hifi companies seem to have gone overboard in sizing it. It is because their job is tougher that it first looks.
Vertical biamping can result in impressive improvements in dynamics and detail simply because the power supply isn't as heavily loaded. I have been designing amplifiers and it is clear the power supply is really what the performance level boils down to.
I horizontally passively biamp and love the results. Absolutely fantastic - and each amp sounds way better than it can on its own (for its respective frequency band, that is). It does make a big difference.
Arthur
