Yo Russ! Welcome To Talkbass.
Nice score. I hope you are bringing that crazy tub to String Summit this year.
Well, you've seen my setup on the Cabletub Bass, in which I use a MM Stingray PU (wired to use only one coil passively). Of course, it gets an electric sound as opposed to the natural 'tub thump, but this allows me to have a more pronounce attack when the band is playing in an electric setting.
The K&K would be a nice choice if you want to get the real resonated sound from the washtub. Or if you want to keep it funky, try mounting a couple contact mics on the tub (similar idea to the K&K, but more lofi). I got a 6-pack of these cool bottlecap contact mics (Red Stripe caps even!):
http://www.getlofi.com/?page_id=1479
or you could co complete overboard and do both and be able to blend between an electric pickup and a piezo/transducer pickup.
And yeah I'd go with some flats. Is the neck fretted?
Cheers,
john/four2oh