Noticed the same thing
Atgatt, I installed my HB bars yesterday and immediately noticed exactly what you described but it was primarily in the range of 2500 to 4000 rpm.
Building on your suggestion, I cut some heavy rubber fuel hose to take the place of the metal spacers used in the rear (bike center) HB mount. Definitely helped but it still feels more vibey than before the HB bars.
During install, I noticed the gap between motor-mount and frame tab and threw some washers in there. I've read this morning that others (Gustavo included) may think that these are unneeded and may even cause some vibes. Indeed, I did do each motor mount one side at a time so may have seen an artificially large gap between mount and frame tab.
I'll try removing those spacing washers today and let you know what shakes.
Oh, and I really like the look of the bars but, damn, are these things heavy! I could use them for doing curls...but I do have pretty skinny arms!