It goes back to my unscientific rating of weight. When a 55wpc HK weighs more than a 120wpc Yammy, then you need to look internally to see whats going on.
Generally, what makes up the bulk of the weight of an amp is the power supply (i.e. toroidal transformer), and the heat sink(s). The bigger the power supply, the more current it will produce, and more efficiently.
The larger the heat sink, the longer the unit can run at reference levels. Heat can shut down even the best monoblocs. That's why these massive Krell and McIntosh class A and AB amps weigh in excess of 100lbs. They have huge heat sinks to dissapate heat. Keeping the heat away from the electronics will allow the unit to run longer with less distortion.
There are currently no receivers on the market, rated with any type of THX certification, that weigh less than 33lbs. THX standards are pretty high, and only those receivers that make the grade can put the THX label on their units. To date, the Pioneer 1015 is the most economical THX receiver you can buy new. THX is not an end all to receiver ratings (other outstanding units don't have this THX rating, and weigh in excess of 33lbs - but choose to save the certification dollars to pass on to you and I).
What is rarely ever discussed is current limiting. Many of these sub $1000 receivers have very similar ratings. Some choose a higher current limiter to protect the unit. This is critical if the heat sink and protection circuits cannot handle what is flowing through the unit. I'm not sure how to find out what units utilize what size current limiters, but that may be of interest in other topics.
IMO the weight of a unit (or THX certification) in deciding upon receivers sure beats the watts per channel B.S. the FTC tries to regulate. It's not scientific, but I feel it works.
