I had problems with my bed vibrating a lot on my Printrbot Plus V2, so I increased the bolt size, spring size, changed the nut to a standoff and created a nice knob to use in place of the wood knobs. Works fantastic now, all vibrations are totally gone in the bed.
First, print off the adjuster knob.
Second, at a local hardware store, I purchased new standoffs instead of standard nuts, this allows for a longer mating surface to the bolt.
Third, I installed new 4mm bolts, they fit very snug in the stock holes, just have to clean the holes up a bit, but you want a tight fit, but not so tight that you can't move the bed up and down.
I also installed new springs I purchased from Harbor Freight(spring assortment box) to allow for longer length bolts and a tighter bed.