We've done low-water squirting on Brown's down to 500-600 level. It's mightly low until you reach the canyon, but then the river narrows a bit and the channel gets deeper. It's not super fun and it gets more picky-technical, but it's still wet. Keep your eyes open for the deep channels. I'd definitely recommend the Ruby Mountain put-in for reduced damage to the hull, since it knocks off a fair distance of low-water torture
in the shallows.
Sorry. I have no low-water advice for Numbers.