Sorry, I haven't been back to this forum in a few days. I still don't know why the hum affected the boost and not the OD, but the trouble was definitely the wall-wart. Here is the/a "fix": I found an LM317 3-terminal regulator in my spare/junk parts bin and "dead-bugged" the basic circuit from the LM317 datasheet (try the National or TI web sites, or Digikey) - it's 2 cap's (0.1 and 1 uF) a 180 or 240 Ohm resistor and a 5K trim pot. Just read the datasheet. No pix, it isn't pretty.
The 9V wall-wart puts out about 13.5V no-load but anything up to 28Vdc input will work and the pot can vary the output from 1.2 V to just less than the input voltage. The regulator gets rid of the hum almost completely - not quite as quiet as the battery, but I did start with a crappy wall-wart.
The LM317 (and 117) are the adjustable versions of the 3-terminal regulators - they come in various fixed voltages as well including 5, 9, 12, 15 etc. They are dirt cheap and if you can BYOC you can BYO regulated P/S.
Next I plan to open up a wall-wart (I have a box full of the things) to see whether I can salvage a transformer and maybe a rectifier circuit - I don't really know what's in them - then add some decent filtering - should be fun.
Thanks for the responses and suggestions - sorry I didn't get back here sooner.