What happens if you lower the output level from the sound card?
I can see a couple of possibilities why it may clip: Sound card output is clipping; sound card input is being overdriven; amp is clipping.
I would be surprised if the amp is causing trouble, though depending on how they designed the circuit, it could. There are a couple of different relay attenuator topologies and at least one of them has variable input impedance. That could end up loading the buffer (or your sound card output if the amp is unbuffered to the volume control) with too heavy a load, thereby degrading THD. If that's the case, you'll find that the amp will distort around certain volume settings but not away from those settings.
Tom