Looking at your schematic;
http://www.freeinfosociety.com/electronics/schemview.php?id=297
It would seem that both channels run through the same tubes, so there isn't anywhere obvious to start as far as which tube is causing the fault. The fact that only the orange (clean) channel is acting up might mean that there is more to this story than a bum tube. Your amp is basically a 1 channel amp, that switches in/out two sets of gain controls and tone stacks. I would expect a noisy tube to show (and even more so) on the high-gain channel.
I don't have any experience with this amp, so I'm just speculating. It could be that one of the LDR switches for the orange side is playing up, causing an abnormal value or even no connection when there should be one, or could be a bad component (pot, resistor, capacitor etc) somewhere on the orange side.
At the bottom of the schematic link I posted, there is a "Tech Bulletin", presumably from boogie. You might want to see if your amp has had these modifications. If not, this might be a good time to get it in to see a tech and do the bulletin updates as well as any other small fix to get the amp back to normal. The Bulletin is related to the LDR switching system, which may or may not be related to your noise issue.
Good luck, whatever you decide. Post the results when it's fixed. you'll be helping the next guy!
