Hi.

I must say yes, the low-impedance is the main reason. Also be sure the K701 sound as always because sometimes the problem could be a tube, a cable and its shielding, the position and distance between the cable and the transformer and stuff like that.

The CSP2 sounds its optimum for headphones with high impedance. Even the K701 is not really a high impedance headphone. I'd say the best combinations are your german Sennheisers HD600/650/800 (300 ohm) or Beyerdynamic (250 ohms/ Tesla1 600 ohm).

As I know very well these sennheiser's because of I actually own them all, they perform quite well with it. They are typically a bit cold, except the darker and warmer HD650, so the warmest possible tubey sound is perfect for them to achieve a good balance. It's of course my opinion.