My fiddle had a strange "woofy" sound when playing notes around the D on the G string.  A different string and bowing changes helped but the tendency was still there a little bit.  I read on another forum how an older, experienced player tweaks that on his set-ups.  He claims that he sets the afterlength of that string to sound a D two octaves and a fifth above the open G.  So I thought, "What the heck, I'll try that".  Guess what, it really helped.  I only needed to move the bridge back about 1mm.  That doesn't really alter the 1:6 string length and it is much quicker and easier than messing with things like the sound post or tail gut.  Also I don't notice any difference to any other notes/strings at 1st position and I can always move it back if I choose.  

I'm reluctant to believe a lot of what I read on the internet.  It has to make sense to me before I try something.  This sort of did and it worked for me and doesn't harm anything, at least if you maintain the correct angle of the bridge to the top plate.