Biamping is certainly useful with speaker-level crossovers, it does not waste any more power than single-channel-per-side does. Of course, as you say, line-level crossovers are better, with good parts, But good active crossovers are unfortunately rare, expensive, and hard to implement. That said, vertical biamping is often a _better_ use of amp power than a typical single-stereo-amp setup. The reason is that each amp's PS feeds a high-current (low frequency) driver(s) and one or more midrange and HF drivers. So the total load on each amp is the same as for each channel of a shared stereo amp. Most amps are limited by either the voltage or current of their power supplies, not by their output devices. All of the power of each amp can be used, except for the midrange and tweeter pads which don't dissipate gobs of power anyway, with most music.Biamping is only usefull when the filter is before the amps, so the amps are direct coupled to the speaker drivers (no filters or only a security cap on the tweeter). Otherwise it's a wast of amp power. It's promoted (biamping on speakers with passive crossovers) by many in the audiophool world altough, but mainly because they can sell more amps that way.
And a well engineered stereo amp with one psu is normally as good as a dual mono like you describe. But then the psu must be more powerfull than what the amp needs and the engineering needs to be right done to eliminate crosstalk, what is often an issue with stereo amps. The builders wants to reduce the costs and so cuts corners to get there, with as result crosstalk that is to high or even worse, sagging of the amp because the psu can't deliver the juice needed. With a dual mono (or monoblocks) you're more sure to avoid those issues, but it's not the only way, and no guarantee to success neighter (even a monoblock can sag).
Most stereo power amps are definitely not as good as dual mono amps of the same rated output: The combined PS in a dual mono amp typically has more capacitance, providing more instantaneous power to each channel. And crosstalk is drastically lower: 20-30dB is common.
So the two main benefits of vertical biamping are 1) much lower crosstalk, which improves imaging; and even more importantly 2) separation of channels between the drivers _greatly_ lowers distortion for the mid and HF drivers that otherwise suffer from the back-emf of the woofers.
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