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 think the issue is with external vibrations, like say banging on a keyboard or walking around near the table where the Crack sits. I forgot that I could not keep the Crack on my desk as I hit the keys on the keyboard pretty hard. I keep the crack on a table next to my desk.
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.
The way I, and I think most others, orient the crack is to have the volume and jack on the front and the inputs and power on the back. While the input tube is a little close to the volume and jack, I wouldn't say I have to reach around anything to use volume, jack, or power.