any thoughts on a bass drone that cuts out when all 3 drones are open, but doesn't cut out when the baritone (E) drone is capped (border pipes, bellow blown)

At a guess it's not getting enough pressure to keep it open. If this is the case chances are that either you're not yet steady enough with the bellows to sustain all three drones continuously or the tongue of the reed has become a little stuck. If you think its the reed try sliding a bit of a stiffish sheet of paper between the tongue and the reed body, if there's muck on the tongue this might help you to spot it or clean it. You can also try giving the tongue a bit of a flick with your finger nail or some such, very carefully so as not to sprain the leaf. If these don't work try edging the bridle down towards the reed seat. Do this in very small fractions of a mm steps until the reed kicks in again.

As a quickie make sure the reed is well seated in the drone and all the joints are good too and not sloppy with dried out loose hemping.

thanks. I have messed with those things a bit, but can try again. When the baritone drone is plugged, I can't make the bass drone cut out...only when I have all 3 drones open. I've tried turning the drones in their seats a bit to change the harmonics, but that doesn't work for long

The fact that when one drone is plugged the bass sounds does make me think pressure. If none of the obvious has worked and I knew I was playing steady I'd be looking at tweaking the bridle. What kind of drone reeds are you using and how old are they?

Back in the day with the pipe band and cane reeds we used to use a hair slipped between the reed tongue and body to prevent things from closing up. I've not really had to do this much since good synthetic drone reeds have been around but it can still be a useful trick and especially help to play in a new reed. It's also an easily reversed indicator before playing with bridle positions.

I think I've come up with a solution, before I start playing I squeeze the chanter reed and warm it a bit in my hand. Then it takes less bag pressure, which seems to prevent the bass reed from shutting down. I am enjoying them much more now, when I get time between shoveling snow.