The tubes aren't entirely orange. It's just the heater part that glows orange, like you said. The tube on the right looks identical to the others while at rest, but when you play a note it goes blue. And it pulses harder if you play a big, heavy note.
The guy we bought it from apparently had it worked on and retubed before he sold it to us. So my guess is that something has gone wrong in the past few months, and thus the noticeable decline in tone. I'll bring it in to my amp guy this week and report back with the diagnosis.