For what it is worth, the resonant frequency of a spring/mass (which is what an isolation mount for a valve is) is given by (1/2.pi).root(g/B)
Where g is acceleration due to gravity (9.81m/s^2) and B is how much stretch there is in the isolation springs. Therefore the resonant frequency depends only on the stretch of the suspension springs.
So if the springs stretch by 1 cm (10^-2 m), the resonant frequency is 5Hz, which looks fine. If they stretch by 1mm the resonant frequency is 16Hz, which is probably still OK.
Mullard did quite a lot of work on internal resonances and hence microphony in a valve published back in the 1962. Download from where I found these
http://www.thevalvepage.com/valvetek/valvetek.htm
Craig