Power Out = I (current) x E (Voltage)
I = E/R (resistance)
I can feel the rolling eyes already
Your measured power out is going to be dependant upon the supply voltages through out the amp and the load (speaker or test load). And since speaker coils are frequency reactive transducers and every one is a little different and geets well...they play all sorts of different sounds testing this could be tricky. To make measurements even more difficult to make, many manufacturers write these specs using a single test tone or a series of test variations which are pretty damn tough to duplicate except in controlled environments with calibrated test equipment and test jigs. A single frequency pure sign wave input might make up for the difference from what you measured and their published spec but someone call me a doctor if we gotta listen to a 150 watt 1kHz tone through a Vintage 30 speaker

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Bottom line, I'm not sure how you made your measurements nor do I claim to know how Mesa developed thier specs but I'm willing to bet that Mesa could post a situational test where their product meets their claims which would "technically" agree with the original Watt's Law. It would be very interesting to see how they come up with this number and compare it to how other manufacturers do the same. Many industries...not just in amp land, do the manufacturer's of similar equipment make it so hard to get to the apples to apples for us consumers. I really hate that!
I've got a tripple...and oh hell!...it's too loud...I recently aquired a Univalve which yes, it's a completely different animal but still very tasty...still plenty of shout for the studio at 15 watts. Like several before have already suggested one way or another...check into a smaller combo but definitely test drive before you buy. If sagging power supply sound is what you're after, keep looking, there are other makers of smallish amps with tube rectifier power supplies out there.
You've got the 3 dB power doubles thing spot on...from this you gather that it's a non-linear curve which explains why a 15 watt amp running at full tilt with a 1x12 speaker is still way too loud to play in an appartment at 2 A.M. which also would support the idea that the difference between 130 to 150 watts is really pretty negligable...it would be tough to hear if at all possible the difference especially if it were a 1kHz tone (thanks to Fletcher and Munson

those guys spoiled it for everyone) or better yet, my all favorite tri-tone interval

. Did I mention that double stops and tri-tones are my big eared beagle's favorite sounds? He can nearly turn his head all the way around when he hears them. What a freaky animal...anyways...peace, love and all that happiness shit.
