" ... 200 watts in 1973 is like 1000 watts in 2010!! ..."
I'm pretty sure 200 watts is still 200 watts. It's interesting, though, that you picked 1973, since that was during a time when manufacturers pulled power ratings out of a hat, and many were wildly misleading. It was not until 1974 that the US Federal Trade Commission introduced the "Power Rule" specifying how amplifier power could be advertised.
Prior to 1974, some used the voluntary EIA (Electronic Industry Association) standard which required one channel driven at 1 Khz at 1% THD. Others used burst tones of short duration, and distortion figures from 10% to full saturated clipping (100%). Amplifiers would be tested at 8 ohms, and then using a formula, not a test, that number would be extrapolated to 2 ohms, with a simple multiplication by 4. And so on.
I remember seeing amps in the Sears catalog with cheezy BSR changers selling for $79.95 and claiming 300 watts of power when I was a kid, and I listened to them at friends' houses, wondering why they crapped out when another friend's dad's amp claimed 10 watts per channel and blew it out of the water.
Although it's difficult to correlate these inflated figures with what the FTC method would produce, it was not unknown to discover the worst offenders claims were 10 or more times higher than the power they could deliver via the 1974 FTC Amplifier Rule.
An example, however, of a FTC rule amplifier using the typical methods, would go something like this:
Given unit A: an amplifier that can deliver 10 watts/ch, both channels driven, from 20~20Khz at 1% THD into 8 ohms.
First we test with just one channel. It manages to do 14 watts.
Then, we test with just an easy 1 Khz signal; now it does 17 watts.
Not enough. Okay, lets move the distortion level to 10% instead of 1%. Now we get 21 watts.
This is the end of the testing.
The rest is just math:
We measured RMS values, let's use Peak values (RMS = .707 x peak). Now we have 29.7 watts. We'll round that up to 30.
We tested into 8 ohms, let's just convert to 2 ohms. That's 30 x 4 = 120 watts. Now we're talking.
Remember, that was just one channel. Add em up.
"Our amp puts out 240 watts!"
Call the ad agency.
The FTC rule requires a specified warm-up period (60 mins at 1/3 power followed by 5 at full power) before testing, and defined the claims advertisers could make. Probably the most valuable rule was one that stated the FTC method power must be the one most prominently displayed in advertising ... you couldn't claim your wild power and then put the FTC number in the fine print.
The warm-up period eliminated specifying power into 2-ohm values by even most well made amplifiers, since the warmup would generally mean the amp would simply go into protection mode and shut down before the 2-ohm test completed. When cold, it may have been able to complete the test.
The 1974 standard required that all "associated channels" of an amplifier must be driven to full rated power simultaneously during power-measurement tests, but Home Theatre manufacturers found a loophole whereby they could define which channels are "associated".
The modern 5 and 7 channel amplifiers sold today broadly speaking cannot deliver full power to all channels at the same time, but might be able to deliver decent power to two or three of the 5 or 7.
Home Theater amp makers took that as an opportunity to rate their amps differently than stereo units. They would test the amps in 2-channel sets; ie the front two alone, the rear two alone, and if a 7.1 amp, the next two rears alone, and finally the centre channel by itself. Then add it all up for an overall power rating.
Currently there is no FTC standard for multichannel amps. The FTC has been proposing one for more than a decade (since 1997), but the industry has stalled the process, and it's still in the comments stage. In 2002, the Consumer Electronics Association proposed it's own standard (CEA-490-A, "Test Methods of Measurement for Audio Amplifiers").
That got the FTC off their back for a while, until the FTC noticed that most manufacturers were not even using that standard proposed by, essentially, themselves. So, currently, the FTC is again threatening to make multichannel amp testing standards a law.
However, it's important to remember that it won't change much ... don't expect a standard demanding all 5 or 7 channels driven simultaneously, with the warmup, etc. Manufacturers have convinced the FTC that this would force them to re-rate amplifiers they are already selling with more optimistic ratings. Expect something similar to the "2-1-2" method.
It should also be noted that a few manufacturers do rate their multichannel amps by the full 1974 FTC method now, with all channels driven simultaneously (eg Bryston, Classe, etc).
The wildly inflated power claims never actually went away ... any "portable device" is exempt; that includes TVs if they have a handle somewhere, car stereos, blasters, iPod music systems, etc. Makers of these devices can still basically claim whatever they want.