Well, not that this adds any value to the cab detection effort, but I had just got off an airplane after a business trip a few months ago, and I was tiredly walking while looking at the ground. I spot a pair of psychedelic Converse sneakers, look up, and it's Rick Neilsen, dressed pretty well the same as on stage. Gotta say, he looked kinda old (but he is 64....).
On the recent Austin City Limits broadcast, it looks like TP was going through a Hiwatt bass cab. On top of it was a Hiwatt combo amp. A little Googling came up with that he usually runs the bass through a guitar amp for distortion as well as the main bass rig.
So, the guess that it's a prop cab on Conan is most likely...
js