One of the first steps is always swapping tubes from right to left to assure yourself that the tube is not bad. In your case, where you think that the Seduction is eating tubes in one channel, you should swap the bad tube to the other channel and see if it is bad there.
If not, your problem is that the bad channel starts to drop volume and goes dead.
BTW, the diode should work or not, they are kind of binary in that regard. Check the other channel HLMP 6000 with your meter. You have a good channel and a bad channel so you can compare the two easily. Do the resistance check on the good HLMP 6000. It should be low resistance one way.
The problem is meters vary with what they indicate. Some have a "diode check," indicated by the diode schematic symbol, where it will read the forward voltage drop, 1.7V for this diode.