Is there a general industry standard for the amount of Class A power in
Class A/B amplifiers?For instance SimAudio has always touted that they
run Class A for the first 5 watts.
No. If the amp is only 1/4 or 1/2 watt before it goes into B operation its still considered class AB. Some amps out there have what is marketed as 'enriched A' operation where they have 20% or so of full power in the A region, but that is only marketing; such amps are still considered class AB. There is no such thing as 'A/AB' unless there is a switch to differentiate the two.