My experiences with Tube Amp's both Pre and Power is that they are bettered in their End Sound when seated upon a Isolated Plinth.
After numerous trials with materials as a Base Board and Footers, the one which to date has made the biggest impression at Tidying up the Sonic being produced, is a Phenolic Resin Impregnated Densified Wood Board, with Audio Technica AT 616 Footers.
A board ss the one suggested is not too difficult to acquire. I am also confident Footers can be found to be equivalent in performance to the 616.
I strongly suggest a support for the Tube Amp is given consideration.