This is extremely important information, as way too many people think that doubling their amp's power output doubles the volume; in reality, there is just a tiny bit more perceived output (3dB). And as agedhorse mentioned, the tonal differences ("baked-in tone") can and does have a very significant bearing on perceived volume levels, too. Not to mention (okay, I'm mentioning it) the relative positions of the volume controls (gain, master, etc.) on the amps have no bearing on how much power an amp is putting out. Tone control stacks are totally different, too, and setting a "flat" response is not usually all EQ controls straight up, or some such. So comparing a 400+ to a 800+ may not be as straightforward as one would think.