It'll definately shorten tube life. Me, personally, I don't find any benefit to running diode mode at any volume. I just find a harsh and abbrasive tone that I can't dial out for the life of me. I really need to get some GZ34 rectifier tubes coming because they are supposed to give you the punch and power of diodes with the warmth and tone of a tube rectifier.
I ran E34Ls for 3 years on diodes, running at 90% dissipation, and didn't have a single problem. I still have the tubes as a backup set. Of course, these were JJs, and JJs are very tough. I wouldn't try it with some brands of tubes.
The problem is that with diodes, you run at around 475 plate volts, and most EL34s aren't designed to handle above 450.
