Pros: Perfectly voiced for the HE-6
Cons: too much gain for some headphones
In the months leading up to the release of the EF-6, I was considering the few options available for amplifying the HE-6, there is basically speaker amplifyers and a few pre-amplifiers designed to power the AKG K-1000, which was prior to the HE-6 the most notoriously difficult headphone to drive on the market. A few things stood out about the EF-6, there was the handsome good looks, the speaker amplifier style case with large heatsinks on either side designed to cool the discrete, class-A biased output stages, all the way to 5 WPC into 50 Ohms. Reading over the spec sheet it was hard to fault the design, small things like the solid looking shields over what I can only guess are the massive transformer and filter caps. Everything just looked incredibly well built and engineered. There are few amplifiers on the market claiming class A operation that actually operate with class A switching for the full power output, EF-6 is not one of these - it operates in class A for the full rated power output.
Upon receiving the HE-6 the first thing I noticed was the weight - it almost felt moderately light when packed in the boxes it ships in. One box reveals another, and that reveals one heavy beast of an amplifier which appears to outweigh it's size. I can say that it looks every bit as good as Iron_Dreamer's beautiful photographs show it to be. Once hooked up to my DAC, the EF-6 takes a pause to switch on. All the connectors and controls feel solid and affirmative in their operation, the gold plated Canare RCA's look great, and the XLR and TRS connectors at the front are recessed into the fascia. For reference this is a single ended amplifier, the 4 pin XLR at the front though allows compatibility with the original HE-6 cable as well as any other headphones terminated for balanced operation, which means no XLR to TRS adapter is needed.
One of the first things I noticed about this amplifier is how hot it runs - it is definitely in class-A but definitely not hot enough to be uncomfortable to touch. After about 100 hours of use I felt confident that the amplifier had indeed fully burned in. The way I would best describe the EF-6 sound quality would be powerful and yet refined. The shy and ill defined bass I had previously come to expect from the HE-6 when used with a lesser amplifier was replaced by deep and powerful yet controlled bass which I never knew the HE-6 was capable of producing. This newfound bass ability of the HE-6 was also coupled with excellent dynamics, percussive impacts are rendered in a more life-like way. The bass and percussion are now able to drive the pace of music. The treble, I found extended and refined without any fatigue, and yet not at all lacking in clarity or definition. Vocals from the EF-6 are rendered with excellent transparency completely free from gloss or glare, just as it should be.
Overall I found the amplifier clear, transparent and yet able to avoid the fatigue typically expected from the HE-6. In fact I would call the pairing one of the more neutral in frequency balance I have owned. The EF-6 is incredibly powerful and possesses a massive amount of dynamic headroom. When I received the unit it was set to -10dB gain on the back, and with good reason. With a source putting out 2.5V (typical line level) I never listen past 11 O-clock on the volume control. I can't imagine anyone needing to use 0 Gain setting with the HE-6 unless their source puts out a very low voltage level.
All in all I thought the EF-6 the perfect compliment to the HE-6 headphone, and I would imagine it would work equally well with many other power hungry headphones. The amplifier is very transparent and refined, and powerful, qualities which IMO should bode well with any headphone. There is no risk of blowing up your headphones - Hifiman have that covered with a gradual power on and power off. With the EF-6 you can be assured that your are providing the HE-6 with enough power to hear it as it should sound, and that there will be no problems typically associated with with synergy or lack of power. I would see little reason to use a speaker amplifier when the EF-6 can extract such performance from the HE-6.