I have twin stereo amplifiers, and in my testing the results were inconsistent. On my old circa-1996 Legacy Focus speakers, which were supposedly about 95db/2.83v efficient, I was convinced I could discern an advantage from passive vertical bi-amping. Not that it was of the order I could pick out by switching, it was a less fatigue sort of thing over an evening. I find I'm pretty sensitive to listening fatigue, and I'll pay to eliminate it. The Legacy's were odd ducks, ported enclosure, three 12" woofers, two 7" midranges run what seemed like without a high-pass filter, a soft dome tweeter, and a ribbon super-tweeter. The measured impedance was quite low in the bass (dipping to less than 3 ohms, if memory serves), and quite a bit higher impedance in the higher frequencies. I really enjoyed them on solo piano, and they could seriously rock. (Too bad I was never happy with the sound on strings.)
In my Salon 2 experiments with the same amps in vertical bi-amp mode and non-bi-amp mode I can't tell the difference. I almost sold the second amp, but now I run a low-pass filtered signal to the Salon 2 woofers from the DD18+, so two stereo amps are required. (The Salon 2s sound better if the mid-high section is run direct from the pre-amp.) The filtered configuration sounds much better in my room than running the Salon 2 woofers full range.
Mostly I think passive bi-amping is for people with more money than sense, unless you use amps that are cheap compared to your budget. Or you can really convince yourself you're getting a benefit, and even then there's a good chance you're imagining the benefit.
