Two amps, two Audeze. With my LCD-2 and LCD-XC, I had open-backed and sealed flavours. Without even thinking about it, I assumed that twin-amp drive would treat both the same. When it didn't, I was flummoxed and hard pressed to grasp why. The short attempt at an answer would simply say that I loved what dual mono did for the 2; and hated what happened to the XC. To appreciate my reaction to the latter, think of its sealed concept as two tiny rooms without any windows or doors. These concrete bunkers attach to either ear like silly cubic protuberances. Now overload them with air pressure. You'd get really bad compression effects. With the LCD-2, the rooms were completely open in the back. This eliminated the compound pressurization effects which wreaked havoc especially on the XC's treble and presence region. The famously chewy Audeze midband overloaded in fully balanced mode when the backs of its diaphragms closed off. Gobs of image density are all well and good but as with anything else, overdone is overdone. Balance and moderation remain key. With the LCD-2, dual mono just pushed the envelope of chocolately richness whilst still closing it off for safe delivery. With the LCD-XC, the envelope bust open for a right mess. In the treble and with my Polish hybrid silver/copper wires, the LCD-2 gained unexpected but well-appreciated energy in mono mode. With the XC, this overstepped into sizzly bright bite and weird careening glintiness which were instantly off-putting and disqualifying. Unexpected but again well appreciated, Audeze's (in)famous bassiness did not go overboard but tightened the reins. This eliminated their elephantine aspects and stepped back into feeling properly awesome. Whether a pure copper leash on the XC might have ameliorated some of my complaints I can't know. Let's simply eliminate the XC from the running for being clearly not synergistic. Let's say just a bit more on the successful half of my 'fully balanced' Audeze experiment.

Whilst all three aspects of the HE1000 test resurfaced, the strongest of the three now was on bass control, punch and articulation. On raw lucidity, separation, detail and soundstage scale, the HE1000 already operate in a higher class. Dual mono merely increased their lead. With the HifiMan, dual mono felt like watching the night sky from our prior high desert digs outside Taos, New Mexico in Arroyo Seco. Air and light pollution were zero. On a cloudless night, you could see stars everywhere. The entire Milky Way was lit up in one giant otherworldly light show. With the LCD-2, that sense of intense illumination and vast space was weaker in general and further so because twin-amp drive pumped up their already dense demeanour. With them the 6th or overdrive gear of dual-mono balanced was about intensifying mass and displacement shove to up the boisterous meaty aspects. The explicit aspect in the detail domain played a lesser role.

In terms of potency—of gains between normal and dual-mono drive—I'd list the HE1000 first and the LCD-2 second. The LCD-XC entirely disqualified for getting far too bright and weirdly 'overloaded'.

Exploiting dual-monaural amplifier drive for headphones not speakers relies on a 'balanced' wire harness. This is to end up with two discrete ends, one for each amp. In the Questyle scheme, those ends are 3-pin XLR. In theory, twin 6.3mm plugs wired up for mono would work the same if your amps of choice were so fitted. 'Of choice' is an obvious euphemism. Except for the $15'900/pr Woo Audio WA-234 and today's €3'000/pr Questyle CMA800R, I'm not aware of dedicated headfi monos. Which isn't to say they don't exist, only that a final count should net well fewer than ten. As it turns out, that'd be no reflection on the scheme's intrinsic merit. Its current rarity will be the case because it presupposes headphones of sufficient statement caliber to warrant; and listeners of a sufficiently extremist bend to pursue in the first place. For example, Questyle themselves had indicated that Sennheiser's HD800 would not really benefit. This leaves me uncertain how to know whether a headphone is appropriate for mono drive or not. The HD800 are widely considered the most advanced dynamic design to market. Whether my good results with two out of three testers were due to their being planarmagnetics I can't say. They simply happened to be what I had the requisite matched cabling for to eliminate wire flavour offsets between stereo/mono leashes.

The benefits I heard overlapped with general lore on mono amps for loudspeakers. Soundstage scale, imaging specificity, low-level detail, separation, density, depth of stage and bass all improved. To what degree; and whether that's concomitant with the cost increase over an equivalent stereo amp; that's the big question. Why complexify our setups if gains are marginal? To put it simple, would one be better off upgrading a €1'500 headphone to a €3'000 one and sticking with a €1'500 stereo amp; or adding a second €1'500 amp to the original €1'500 headphone for dual mono drive? My vote would be squarely for the better headphone. Two contributors already—Dawid Grzyb and John Darko—have acquired HifiMan's HE1000 for personal use. Dawid sold off his beloved HD800 to finance the deed. John first compared the HE1000 to the half-priced MrSpeakers Ether before committing. Once he brought home his trophy, he thought his Audeze LCD-2 a bit of a come-down. Their decisions confirm my own HE1000 assessment; and that headphone quality trumps today's stereo/mono consideration. Approaching this the other way—costlier stereo amp versus more affordable mono amps—I'll say that my premium Bakoon AMP-12R beats a single Questyle. With my two headphones however, I'd give the nod to two mono-strapped CMA800R. More than fringe benefit is that two of those plus balanced cable still sneak in at half of what the Bakoon demands. What isn't clear? How much of this is down to Questyle's four-times power increase and distortion reduction when set to dual mono; and how much to just driving each chanel from its own amp. When you think on it, the latter goes fully counter to crossfeed circuits. Those very deliberately cross-bleed data from left to right and vice versa to get closer to regular speaker listening. Meanwhile dual-mono headfi mode is the hardest possible channel isolation. Those who find headfi unnatural might find this even more so. Suffering no such neurological (mis)wiring—I find headfi most certainly different from speaker listening but just as compelling if in its own way—I took a real shine to what two Questyle amps did for the LCD-2 and particularly the HE1000. If I were a mastering engineer, I'd investigate that latter combo in a real hurry. But it's also a terrific audiophile education tool on what's really on our recordings; and what the proper recorded tonal balance should be without any room interactions. Finally, I found dual-mono drive a deeply immersive heightened-state type listening experience and as such, very addictive. Mind you, with the right headphones. With that conditional clause and no other experience beyond my testers, pass it around then...

Questyle website
Forza Audio Works website