I’m not familiar with sound blaster cards but it sounds like your recording a mono signal into a stereo track considering the art tube pre is mono as are most mic pre’s. Make sure to configure the track to be recorded on as mono. I could help more if you at least tell us what program you’re using.
You are probably using the "Mic In" input. You need to use the "Line In", which is stereo. As your mic is mono, you will need to plug it first in a stereo device, as a mixer. Then you plug the mixer in the "Line In" of your soundcard.