Geez what to say about the two...
They share similar characteristics surprisingly, The Ad200 almost sounds like a "half brother" to the Thunderpants.
The Thunderpants beats the Ad2000 in just about every characteristic but the Ad2000 faired well against the mighty TP. I like to think of the Ad2000 as a ninja and the Thunderpants as a Samurai.
The Ad200 is lighter, quicker, and punchier and the Thunderpants is stronger and uses more impact with each strike.
Anyway on to comparisons that actually make sense...
Bass: Thunderpants wins in quality and quantity without a doubt. But interestingly enough, I found the bass on the Ad2000 to sound similar but just with less impact.
Mids: Here is where things get interesting, I think it comes down to matter of preference. If you like airy open mids the Ad2000 is a winner, if you more enjoy the closed thicker and more impactful mids the Thunderpants would be a clear winner. For me, It's tough to decide which one I more enjoy. I feel that some songs complement airy open headphones and others complement the closed better.
Highs: The Thunderpants is a clear winner here, Sometimes the ad2000 can sound slightly sibilant due to its extreme detail. I find this to be a problem with most open headphones. The Thunderpants is just smoother and more laid back, allowing the Highs to be less harsh.
Soundstage: Obviously the Ad2000 is the victor here, but not by much. The Ad2000 is known for its up front mids and slightly recessed soundstage. I thought the TP's did well in this department but if was just well for closed headphone standards.
Side Notes: The Thunderpants have this amazing ability that produces mindblowing sound with Live recordings. I've said it a few times on this thread but I really can't say it enough. On a nicely recorded live song the TP's just sing. Overall I'd pick the Thunderpants over an Ad2000 anytime, except for Classical. For me, I really need that open airy sound when I'm listening to something with a lot of instruments.
Honestly I liked my Bass light version a little more than my Bass heavy that I have now. It just sounded better overall, the only thing it lacked was Bass impact. The bass was much like the bass in the Ad2000, so it wasn't bad, just not that of an average "Thunderpants standard".
My god that was a terrible review but hopefully that will tide you off for now Napalm.