The M1 HPA was one of the first high performance headphone amps at the dawn of the headphone revolution. It sold for $799 in 2011 when I purchased it, is all Class A, and even includes a USB DAC to complement the line level input for additional flexibility. The HPA1 holds it's own with newer head amps even in this retail price range, let alone the current used cost. I've listened to dozens of them over the last 2 years in my quest for an amp with balanced inputs and outputs and had to consider much more costly options to make a noticeable improvement. It's in perfect condition and has worked flawlessly.