Thanks to all for the replies and thoughts on this. This site is a really wonderful resource. On this thread, and after a few years of intermittent doubling myself, I've come to the view that there are many ways to solve this problem. If I were starting from scratch, I would probably drop $600 for the Basswitch IQ DI. Or maybe save a few bucks with either the Radial Bassoon OD or Bassoon V2. Very simple, elegant, and flexible.
But I have a lot of gear lying around the house already, and my practical issue at the moment is that I'm providing the bass rig for an outdoor festival later this month, where we'll have 15 acts and a range of bass inputs. So my solution will be to (1) pull out my 10 year old Radial Bassbone/Tonebone to serve as the basic A/B switch and direct box (and get a new wall wart since the original is torn and frayed), (2) run the electric basses upstream through a SansAmp bass driver and into channel 2 of the Radial, and (2) order an Fdeck series 2M to put upstream of Channel 1 of the Radial. I will run all of this into my Aguilar AG500 and a couple of AG GS112s (starting to seem vintage by now). So for $110 bucks I (a) get a new Fdeck (paying homage to my GAS), (b) overcome the limitation of the original Radial Tone Bone for URB, (c) have plenty of tone shaping for the electric basses with the SansAmp, and (d) go into the situation with a lot of confidence that -- as long as I have fresh batteries in the Fdeck and the SansAmp -- I'll have a bullet proof rig that will sound good all day long.
Comments and suggestions would be most welcome, as always.