Bought this EF600 as an open box and it arrived in superb condition - you would not have known it was an open box. All the cables were included and packaging was as new. A brilliant saving on the new box version!
Currently using them with HE1000 Stealth headphones, which have a slightly less bright reputation than eg HE1000 Unveiled or HE1000SE. The EF600 does not sound clinical or reference, it is more warm and smooth sounding with none of the harshness usually associated with brighter or reference amps.
The EF600 is a very powerful amplifier which has ample power to drive much more difficult headphones than my HE1000 Stealth. The soundstage is very good, but not stunning in a reference sense. My subjective feeling is that the frequency curve is very well balanced. Bass response from this amp is stunning and the combination with my HE1000 Stealth headphones makes it superb for anything with any level of bass, whether it is EDM, organ music or film soundtracks.
Currently using low gain mode, with the amp volume never having to exceed 11 o'clock - and I listen to music fairly loud.
Excellent input/output options inc balanced XLR and RCA line out for use as a pre-amp. Very easy to control using the front panel selectors.
Hifiman headphones have a reputation for sounding bright, but with the combination of the HE1000 Stealth and the EF600 I have not had to EQ my headphones at all, as to my ears they sound perfect with no adjustment needed to the treble.
Only two very minor negatives:
1 - the On/Off switch should have been at the front of the amp - it is a pain to have to reach around it every time, navigate the power cable and then find the on/off switch at the rear. If Hifiman ever redesigns this amp, please put the power switch on the front!
2 - Very occasionally I can hear almost inaudible electrical interference due to insufficient electrical shielding? (the black part of the amp is plastic not metal). Mine sits near a mobile hard drive, so when I have no music playing, I can from time to time hear a very low level of electrical interference through the headphones. When music (or anything at all) is playing at any level, the intermittent interference is totally inaudible. During the writing of this review I had my headphones on the whole time (with no music playing) and I heard zero electrical interference, so I fully expect this interference is entirely from when my mobile harddrive is active and in use - so clearly I need to move the hardddrive to a more sensible position and away from the EF600 amp!
Overall, the EF600 is very highly recommended. It has huge amounts of power to drive your current headphones and your future headphones. It's got a great sound and simply never runs out of puff, whatever you have playing.
