Audition recording distortion, atypical wave patterns

Okay, I've had a careful look at the waveform now. Let me draw your attention to one part of it (the end bit - the word 'three' amplified for clarity):

What you are seeing there isn't a grounding issue as such, but a pretty large shift in the DC level of the output, looking for all the world like a damped LF oscillation. This is not that unusual to a small degree with valve mics, but this is definitely too much - it's shifting about all over the place. So I looked up what others have reported about these Warm Audio mics, and it would appear that you're not alone in having difficulties, especially with the valve in them - they are a bit notorious, as are a few of the other components. What is a little bit strange though is that this is a transformer-output mic, and the transformer should isolate anything with a frequency that low from the valve itself - so basically this is far from clear, and as far as I can go without having the mic itself to look at.

The good news is that most of the comments seem to indicate that W.A. are generally pretty helpful about issues with these mics, so it's definitely worth contacting them to see what they say.