Back in 2001 or 2002, I did passive vertical bi-amping with Musical Fidelity A3cr preamp and two power amps. These amps got great reviews, the only real shortcoming being lack of power (at 120 watts per channel, and they were not "stiff" watts). To address this in response to the earliest reviews that mentioned the lack of power but otherwise superb sound (especially for solid state), the manufacturer made a change in production and built a line level pass-through into the amps. You could thereby daisy chain the amps for easy vertical bi-amping.
I was using Sonus faber’s original Concertino at the time, which were lowish in sensitivity. They really came alive with the extra power, and I was surprised just how much clean power they could handle in this configuration. (In the early days, Sonus faber didn’t even make tower speakers. Bi-amping their refined bookshelves with monstrous amps was kind of a thing that yielded shockingly big sound from tiny enclosures. This practice reached its zenith, at the time, with the original, if no longer "bookshelf" reference Extrema stand-mounted design.)
I also used B&W 602 during this sojourn. These were higher in sensitivity than the Sonus faber, but the old B&Ws had a way of drinking up all the power your amps could muster, and still wanting more. They sounded great with less power, but were capable of much better with more power still. (Not sure whether this has changed with the latest D3 versions.) Unlike the Sonus faber, the 602s didn’t sound good at low volume levels, losing too much life and subtlety. However, with bi-amping came life at low-level listening through the 602.
Now I’m using Musical Fidelity A308cr components, which have been my reference for well over a decade. These were big brother to the A3 and A3.2 series, predecessor to the Tri-Vista kW components, and some of the best solid state Musical Fidelity made before production shifted outside the UK. These amps also have line-level RCA pass-through to allow easy passive vertical bi-amping. But who would need to bi-amp with 250 watt amps?
Well, I recently fulfilled a long-standing ambition to bi-amp with two of these 250-watt bruisers, using JMLAB/Focal Electra 915.1 tower speakers. The speakers are rated 91.5 db sensitivity, so don’t need a great deal of power, except they drop to 3 ohms in the bass, typical of JM designs. The A308 amp never wanted for power with the JM speakers. I wanted to try it nevertheless.
I expected the volume to sound twice as loud and wondered if I’d have to use the preamp at the lower end of it’s volume control. What I found instead was increased subtlety and finesse, with greater detail, larger soundstaging, and more density and complexity to the sound. It was not a knock-you-over-the-head kind of difference in loudness, drive, or bass control, as it had been with the smaller A3cr amps, and bookshelf speakers disguising deceptively demanding loads. It was more a difference of greater refinement and naturalness; an ability to see deeper into the soundstage. I presume the difference is because the amps are coasting in this configuration, and maybe mitgaing the EMF of the woofer in a vertical bi-amp. I use identical interconnects and speaker cables.
I couldn’t say whether they’re harder to find than the Pass Aleph, but the A308cr amps are no longer plentiful on the used market. I waited years until I found one in suitable condition and had spare funds for it.
My experiences with passive vertical bi-amping, therefore, have been entirely positive. However, I have done it with identical cables and identical amps, with speakers designed for bi-wire/bi-amp, and with amplifiers whose manufacturer made provision for easy vertical bi-amping (rather than needing a preamp with multiple outputs).
Speakers have generally gotten easier to drive in recent years, and many manufacturers have stopped offering the bi-wire option, which used to be de rigueur for "serious" designs in the ’80s and ’90s. B&W have long been known to benefit from bi-wire/bi-amp setups, as have the old Sonus faber designs. I find it disappointing that Focal does not offer this capability on its current lineup. In terms of semi- affordable statement speakers, my experience suggests a big speaker like the Sopra No 3 could really benefit from having its woofer driven separately, especially if you’re putting most of your budget into new speakers and can’t afford the tariff for a high quality 400- watt amp. It’s an expensive loudspeaker. Why can’t the manufacturer offer the courtesy of bi-wire flexibility? For many of us, it’s easier to bi-amp with a less expensive high-quality design whose sound we already know than trying to find a more powerful design that retains everything we enjoy about our current reference. This is one reason I’ve stuck with my older Electra speakers and A308 amp, even though I might otherwise aspire to a Sopra design or the Musical Fidelity kW 750 (which sounds decidedly different from the A3, A3.2 and A308 series).
