As the valves are fitted horizontally, I installed some silicon rings on the ends of the valves to support them. I fitted them in the same place that the adhesive had been used, this avoided blocking any of the ventilation holes in the PCB. So they are not really being used for microphony damping in this location, just used as supports. The clearance underneath these particular valves is 3.5mm and the silicon rings are 5mm thick so I cut a small flat in one spot, reducing the thickness to 3.5 and  positioned this on the underside, worked a treat.