George,
Are you absolutely sure about that? If you're talking about tube preamp, I could probably see that as being true because of the nature of how tubes work. However, solid-state preamps do not necessarily have to be biased into Class A mode. It's a specific circuit design that does this. This is true for both monolithic op amps and fully discrete analog circuits. I have found that if the manufacturer has not specifically stated that their device is Class A, I will assume it's Class AB.