Sounds like a bad power tube. Tubes are fragile by nature, it's why Boogie waranties them for 6 months.
Normally the power tubes are paired up as inner and outers; ie, if the power tubes are laid out 1-2-3-4 numbers 1 and 4 are the outer pair while 2 and 3 are the inner pair.
In 5w mode the amp uses one tube from the inner and one tube from the outer pair. One is the actual tube being used, while the other is being used as a dummy load to balance the output transformer. Unfortunatly, I don't know which two tubes are used in 5w mode. You could try powering up the amp while looking at the tubes to see if one of the tubes redplates (you'll see the normally grey plate structure on the tube glow like a toaster element), if you see this power down the amp and replace that tube with a new one.
note: purple/blue haze is normal, and there is a small heater at the top of the tube that's supposed to glow... it's the large plate structure that fills most of the tube that you're watching to see if it heats up and goes red.