erik hansen's marshavalve tube v.o.2 debugging question

[reverberation66]

  I just built this marshavalve tube powered od using mihkel jahn's layout from the layout gallery here.  It works and sounds pretty cool except I get this fairly prominent background hum when the effect is engaged and also a kind of vibrato effect when I play the A string on 6 string guitar.  Seems to be the only string this happens on.  It sounds kinda cool really but I'd rather it not be there...the hum I definitely would like to get rid of.  I built it pretty much according to the layout which seems to match the schematic in erik's gallery.  The only change I made was to replace the 100k trimpot for a 100k audio pot.  I'm not sure what the correct setting for this pot is, it's a bias pot and depending on how it's adjusted generates more or less distortion and background hum.  I've found a few good spots where the sound is pretty good but still more hum than I'd like.  Anybody else built one of these or have any suggestions?