I like to see the tubes exposed in the power amp. It is not critical in the preamp. jmcgrogan2 nails it pretty close, it does possess that relaxing warm feeling like sitting by the campfire. A chrome chassis that reflects the light from the tubes is the best. My VAC preamp is totally sealed, not even a vent slot to peer through and the Manley preamp has a mesh wire covering that you can glimpse some tube glow. My VAC PHI 110 has the tubes encased in glass which looks pretty fetching. Given a choice between two power amps that were sonically similar and one was encased and the other had exposed tubes, I would choose the amp with tubes showing.
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.