It's an inline opamp based analog EQ. That's extra crap, not necessarily good crap, in the signal chain. While I can't be exact at this time, the Zen CAN approach to the HD6XX is a light touch that lifts the sub and low bass, and nudges up the lower highs. I tend to prefer the light touch approach because of the difficulty in getting exact headphone measurements. Really, all the HD650 needs is deeper lows and being just a touch less laid-back in the highs. (I was actually relieved when I saw the EQ curve for the ZEN CAN because I always worry about heavy-handed anal-retentive EQ. EQ can only do so much.) Also, not really in the same spirit. It's an extra fricking box in between the source and amp; and additional modules are going to cost. Better off with a inline digital EQ like the DEQ2496 where we can store dozens of headphone profiles, all customizable, for a hundred bucks more. Actually less because any new profiles would be free. Or just Zen Can with HD6XX and not worry about it, which I believe is the point.