Looks to me like Ampeg endorsees know when to jump off a sinking ship
On Letterman tonight, Will's custom Ampeg rig was fully visible in close shots while musical guests John Hiatt and Lyle Lovett were performing. I think it was a new show, i.e. not a re-run.
Maybe I saw it wrong, but it looked there were no leads going into the input jacks of the B-15 copy that Will uses for a head. Maybe it's just for show? It looked like he was playing his new signature bass, too. The sure way to tell is to count the frets on the neck, but I'm too lazy and/or unconcerned to to it.
FWIW, I've seen him use Hartke and GK stuff on non-Fab Faux club dates, as recently as a year ago.