I had some K1000's a few years back and tried them on my SEX amp. They sounded good, though it seemed to me they drove the amp to distortion at higher volume levels (I can't recall now, but seemed like it was at less than 50% of the rotation on the volume pot...maybe at about 12 or 1 o'clock with zero volume down near 7 o'clock). Depending on one's desire for volume, the SEX was adequate, but only just. For me, there was enough volume below the distortion level.
In other words, even the Crack might have enough power, depending on what level of volume you like. For me, I just never quite took to the K1000's like I did to the Sennheiser 600's and sold them.