A few things
1) The position of the master volume knob means nothing. It all has to to with the taper and design of the volume pot. Some amps, like my TecAmp Puma, don't even start to crank until that master is at 2 or 3 o'clock (the 'audio taper' thing). Some amps, like some of the GK models, get super loud within the first 25% of the volume taper. Other amps like Markbass are kind of 'in the middle', reaching full volume when the preamp is set just below clip at (IMO and IME) about 1 o'clock before power amp limiting was put in place.
2) You hit the nail on the head with companies presenting power specs the same way. Remember, the Markbass hit 405 watts at 1% THD in the Bass Gear magazine test. If you remember that test, the massive GK1200 only generated a bit more at that same THD level. That doesn't mean it isn't louder, or even that it sounds bad, just that the volume is obtained with higher distortion (my guess is, at 2% THD, the LMII would test to the 500 watts it is advertised at putting out

. That's one of the reasons why a 300 watt SVT Tube amp can sound like a 1000 watts... IF you let it grind and distort and 'exceed its rated power level'.
3) The Ampeg hybrids are notoriously 'soft' for their power rating. That could very well be that they use more stringent and more 'honest' rating methods (or they could just suck... I have no idea, but agree that they sure don't perform like the numbers would suggest).
A good example of what I feel is an 'honest' way to rate power is the TC RH450 head. It gets STUPID loud.. much louder than my Markbass heads at approximately the same wattage. However, it does it similarly to the Genz models (based on what I'm hearing, and the power amp limiting descriptions provided by both companies) of using that tube emulation power management circuitry to allow it to increase in distortion and grind (just like a tube amp... or at least kinda like a tube amp) and get louder and louder and louder, at the expense of fidelity (versus, for example, the Thunderfunk or Markbass model of not allowing the amp to get any louder after a certain power level at a certain distortion level is reached). However, they rate the amp at a fixed distortion level for 'clean operation', which seems to be 450 watts, even though the amp gets much louder than you would think.
Interesting topic. Of course, it is rare cab that can use more that 500 or so watts anyway, so kind of a moot point once you get passed that with all but the lowest SPL, largest, highest quality boxes.
IMO!