Idling current in any amplifier is a balance between tube life, reliability and sound. Some makers and some listeners want to run them hot as possible. This reduces the life from 10,000 hours to 1,000 hours to as little as a few minutes. Maximum dissipation is the maximum the tube will experience when the line is the highest, load is the worst, etc. When a designer or listener takes his typical and puts it to the max he has made a big mistake. This "mistake" goes back to the 1930s when RCA realized that designers were misusing the max and severely shortening tube life. The tube makers then instituted "design center ratings" so that short sighted engineers would back off from the maximums. In effect the tube makers did the math that the designers should have been doing all the time.
All that being said. I run my tubes at 50% of max dissipation thus giving a lot of grace for situations where everything conspires to overheat the tubes. I round B+ up rather than down and choose round numbers I can do in my head. The B+ in all the RM-9s is 450-475 V so I call that 500V. At 30 mA per tube that's 15 watts. EL-34s are rated at 25 watts dissipation and KT-88/6550 are rated at 40 watts so you can run those at 40 mA safely. Whether the higher current sounds better is entirely up to the listener.
Here I will note that my amplifiers are designed to sound good with these bias levels yet other amplifiers may not. Some designers use high bias current to cure sonic problems. I get good sound in other ways so I can run the tubes cooler than others do. Have a look around at other amplifier bias specs and see what you find.
