After you have co,pleted the room’s acoustical treatment, bi-amping, real bi-amping with a line-level crossover before the power amps can make a huge difference, both in allowing amp matching (transistors low, tubes high, e.g.) but in dynamic range - up to 6dB.
Another improvement, arguably the most audible, is the improvement of LF control due to the elimination of any low-pass series inductor(s) from the passive crossover. Never was that made more clearly than when Henry Kloss introduced the rare-as-hens-teeth Powered Advent. A stunningly competent speaker, on comparison with its passive stablemates, the advantages of bi-amping were immediately apparent. Much tighter, more impact full bass, a clarity previously nonexistent in the midrange, and the overall voicing of the Advent - neutral to slightly warm, no nasalness or honking on vocals or midrange - still intact.
Upstream from that, the spendy ADS 1020 Tri-amped 12" 3-Way, put on a masters class in what a box speaker could do.
Bi-amping Altecs and JBLshas been a standard practice in studio, theater, and live sound since at least the late 1960s.
And finally, the Magneplanar Tympni III, preferably powered by ARC tube gear, set a high-water mark in soundstaging and resolution few speakers today even approach.
That was 50 years ago, so bi-amping is not news. And whenever I see a manufacturer introduce a new bi-amp speaker, I do tend to sit up and take notice.
But there is a problem. I see lots of people thinking they are biamping by using two amps and running into an existing speaker’s passive crossover biwire connections. That absolutely is not bi-amping. It is just the most expensive form of bi-wiring. It is pointless and wasteful, as it accomplishes none of the goals of bi-amping, yet still manages to double the price of amplification. It feeds full range signals to the high end, where the passive crossover is left to absorb the LF energy and doesn’t remove the series resistance that destroys LF damping, and because it doesn’t separate LF and HF, into separate loads, it does nothing to increase a systems dynamic range.
So, done right, bi-amping can be great, and I am a big fan.