Best way to dampen a micro-vibrating Printed Circuit Board inside an amp? Thanks!

Silicone Solutions SS-101......I have also used certain Silicon caulks by Dap and GE clear (applies white, becomes clear when settled). Another is Frost King Rubber Pipe Insulation Tape. And, of course, Dynamat, Peel & Seal roofing repair tape to do the entire chassis/horns. It all damps very well, when used properly. My best, MrD.

I modify, dampen quite a lot inside a chassis, even those with point-to-point wiring. I start with isolating the onboard power supply transformer from the chassis, as all transformers vibrate / resonate, which is one of the reasons transformers are "potted" in some designs. I use many different materials, such as a Dynamat type product and some heat pipe tape. Types of silicon are also excellent. Ignore those nonbelievers. Amazing what chassis damping can do for the performance. Enjoy! MrD.

Yes, it all matters, for the better. There is much available to read on the subject. Microphonics is for real, and taming/reducing/eliminating them, is audible to my ears, in every case. Whether it is measurable, well, this is a common argument that continues to go on forever. Measurements vs hearing? Enjoy! MrD.  

@davidplombier, I agree completely, as the smell of the acidity release, it is not pleasant......but after the curing/drying process, it becomes quite safe to use.  Excellent ventilation required during the application process. Almost 60 years, never an issue as a result of using it. Doors, windows, tubs and showers. It is used because it is faster drying, and yes, less expensive. It gets the job done, with a smile on my, and others, faces.