I was doing some research (looking for reviews) on a amp (Blue Circle BC28) that a coworker was considering purchasing from a audio shop in Amherst MA.  one of the "on-line" mags did a review in 2003 and had a lab test the amp with the results to compliment the reviewers article (which was positive by the way) .  In the lab test findings they noted that the amps output stage, while resembling power transistors, were actually 4 pin devices, which (they conclude) if you include the case, suggests a power opamp.  I can think of only one device that meets this criteria. ( LM12) The test report is available here: Blue Circle BC28