In my opinion, a properly functioning tube should not be microphonic. The 12AU7 input tube contains all the gain so it will be more susceptible, but there's nothing about the Cracks's topology that makes it more sensitive to microphonics. I have probably a dozen input tubes and maybe 1-2 of them are microphonic.
I do agree with this statement, if you look at the other components used, the wire etc., there is very little in the design that can cause issues. There is some thing called a flag pole effect, if you have a tube that is slightly microphonic sitting on the cabinet I can imagine the microphonics being made evident with a slight cabinet shake.
I had not even purchased a crack, I glanced at a photo of the parts in the kit and I knew immediately I did not want to use the top plate. I went so far as to ask the dimensions and order an 1/8 inch copper plate before I even received the kit. I also incorporated other changes, a larger cabinet to allow for more expansion, dual volume controls, etc. I believe Allan was kind enough to tell me it was a 6x10 inch top plate and I went from that.
I did use the aluminum plate on another build but only after I added a layer of carbon fiber and double the basic cabinet size, two more plates and 16 layers of laquer so it has some extra weight as well, and little vibration.
I should also mention, and this is more of a personal thing, but I did not like the idea of reaching around the tubes on the top plate to turn the amp on or plug in a cable, or plug in the quarter inch headphone connector. All inputs on mine are on the back, controls are on the front, and my headphone jack is on the side in one inch oak. I do little to add any vibration even if it is just adjusting the separate left and right volume controls.