Tubes are generally more prone to microphonics when they are exposed. For this reason alone, they should be inside the chassis.
Manufacturers don't get it. make tubes visible.
After all of this time, why don't manufacturers get it? Everyone I talk to who owns tube gear wants to see, at the very least, the power tubes while the gear is operational. Many want to see the preamp tubes as well. So why do so many great manufacturers still bury the tubes inside of the gear? Are they afraid we will electrocute ourselves? And, if this is true, who is going to miss a few deranged audiophiles anyway, not to mention their wayward offspring and badly trained pets? Interestingly, large transmitting tubes which pose, by far, the greatest danger, are almost always out in the wind.