How to deal with microphonic 300B tubes.

I have a used set of Cary CAD-300SE LX-20 mono amps, and I just love them with my Coincident Victory speakers. But I occaisionally get a weird, intense ringing microphonic from them, especially noticable when an FM announcer's voice hits just the right pitch.

I have them just sitting on my tiled floor, no attempts at vibration stands yet. I'm sure this could help a bit, but I am also thinking that tube dampening could be needed.

Any suggestions? Are there appropriate tube dampening strategies for 300B's?

Thanks,

Jay