The blue hue is normal. The heater glow should be more yellow in color, not red. If the plates are starting to get too hot they will start to turn cherry red either at the seams of the plates or at the center of the plate surface. Power up the amp to warm up, you do not need to plug anything in, take the amp out of standby, you will see some blue hue but is harder to see a lighted room. Turn off the lights. It may look like my avatar. That is normal. More so for a 6L6 tube. However, if the tube does not have that blue hue, it does not mean it is bad or not working. If the tube has any slots or holes in the plates, you can look into the tube and around the orange glowing cathode will be a blue corona, that is caused by the electron emissions. Blue glow is normal. Not to worry. If it is purple, then you need to worry. If the grey or black plates turn red, the tubes are either running too hot and not suitable for the fixed bias of the amp. I had that happen with new tubes already when I had bought them from other vendors. That can happen. Contact the vendor and tell them you got a bad tube in the batch. They should have written a number on a label or the box at least. If it is the measured plate current they may be able to send you a replacement. I would not mix tubes separately as that could place the amp in an unbalanced state. Keep the quad of tubes in pairs. the outer tubes are one pair and the inner tubes are one pair. I am not sure how the Dyna-Watt circuit works. There may be some mention on tube replacement in the manual for the amp. It looks like the outer pair are used for the 25W and 10W modes. So if the amp was in 25W or 10W setting, the inner pair of tubes will not have any blue glow to them. At 35W mode, they should be but may vary based on the tube. Sometimes the glow on the glass is due to something on the glass surface. There will be a blue corona around the cathode at the center of the tube, it is just not easy to see it.
