Jazz: I read somewhere on the web that owners of the Bel Canto EVO amp have had good results damping the sinks as well as other parts. The info may be at Audio Asylum (though I am not certain) and there was mention of specific materials that worked well that you may want to try.
Ringing heatsinks
One of my current amplifiers is a Classe' CA300 and while performing a cable swap, I accidently hit the heatsink with my finger nail and it rang like a tuning fork. I tried the same manuever with an older Classe' DR8 amp and it didn't ring like that. Then I wondered what effect this had on the sound of the amp since the transistors are mounted to the other side of the heat sink. I inserted a rolled up towell on either side of the amp between the shelf and the heat sinks and on top of the heat sinks rested some pieces of mdf wood to dampen the ringing. This does not pose a heat problem with the CA300 as the fins run pretty cool. The sonic change is very noticeable eliminating a glare in the treble and a vast improvement in focus through the vocal regions. The overall presentation is smoother and more lifelike. I'd like to hear from anyone else with a CA 300/400 or another brand with ringing heat sinks as to whether they find any improvement in the damping I've suggested.