O.K. Someone IS missing the point/could be me 
/. My understanding is that in a PARALLEL crossover config. speak. drivers with it's crossovers will be completely isolated from one another. In B&W case mids with highs can be isolated from the bass drivers.
That's correct as far as it goes.
And biamping - passive biamping - WILL make the speaker's load on an amp MUCH lighter.
That's generally
not correct.
The current that the drivers suck out of the amplifier is highly asymmetric. Most of it is drawn by the bass drivers.
Suppose we're driving the speaker with a 200W amp. The bass driver will typically be sucking up about 150W, and the treble and midrange will be using about 50 between them. If you bi-amp with a pair of 200W amps, the treble and midrange have now got 200W to play with, but that's
far more power than they would ever actually need. The bass drivers, on the other hand, have only gone from 150W to 200W - not a noticeable difference.
By contrast, if you use a single 400W amp, that might be split 300/100 in favour of the bass drivers, so
both sets of drivers now have twice as much power to play with as they had before, and both will sound better - particularly the bass.
It is
possible for treble/mid quality to improve when bi-amping, if the bass driver is thrashing a single amp so badly that even the treble and midrange part of the signal gets distorted by lack of current as well. But this is quite unusual. And, even if that does happen, you aren't helping the quality of the bass by bi-amping: that will still clip, and still sound terrible.
In cases where bi-amping does help significantly (such as the PMC speakers biovizier mentions) you're solving a different problem. With B&W speakers the main problem you have to solve is usually "can we supply enough current, especially to the bass drivers?" Giving them enough current tightens up the bass and makes it sound better controlled, while also allowing the treble and midrange free rein. The limit on the sound quality in the PMC case is "can we stop the inadequately damped bass drivers from overshooting and shoving back emf across the treble and midrange drivers?" The best way to do
that is to electrically isolate the treble and midrange drivers from the bass.
Which strategy is the better one varies from speaker to speaker. If the limit on sound quality is a lack of current, then you need a bigger amp, and bi-amping with two smaller ones won't help as much. If the limit on sound quality is a lack of isolation between the bass and the treble/mid drivers, then bi- or tri-amping will be more useful.
Clearly, if you have money to burn, then bi-amping with 28B-SST amps will sound better than single-amping with 28B-SST but the difference (on B&W speakers) will be small. And, in the majority of cases, it's better to use a single large amp than two smaller ones. As James T says, you can sometimes get good results using a large amp for the bass drivers and a smaller one for the treble and mid, but that's not something I'd advise you to mess with unless you really know what you're doing!
The one case where bi- or tri-amping
really sounds good is when you can
actively bi-amp, i.e. when the amplifiers are in between the cross-over and the drivers, rather than in between the source and the cross-over. But obviously a speaker has to be specially designed to allow you to do that. (The original B&W Nautilus speakers, for example, have an external cross-over, and require four monoblock power amps per speaker).
