Brad, with the autoformer, isn't the mc2300 designed to deliver 300watts at max power into any ohm? and in fact that is a conservative rating as most mc's will perform higher then their rated output?
No. You still have to match the speaker impedance to the correct output tap on the Mac's autoformer to get rated power. What's different here is that on a normal (non-autoformer) amp you can't get full power at various impedances, only at the lowest impedance allowed. The autoformer lets you use various speaker loads and get full power with any of them, but you still match the load to the correct amp output tap. If you connect an 8 ohm speaker to the 8 ohm taps, full power. If you connect a 2 ohm load to the 2 ohm tap, full power. On a normal amp, the lower the impedance load of the speaker, the more power you can utilize. An 8 ohm cab on a "normal" amp will not take full advantage of its available power if that amp can also drive 4 or 2 ohms.
But this is often confusing with the Mac's. The autoformer does NOT automatically deliver full power to any load. For example, if Jerry's 3-JBL cab was 2.7 ohms and they tied it to the 2 ohm tap on the Mac, then that mismatch, like any impedance mismatch, will reduce the available power to the speaker.
Rumor has it that Jerry actually did have a 4th JBL connected, but it was on its own cord and was on stage behind the drummers. I guess for the crew's enjoyment, or maybe so Parrish could keep a close ear on his rig in case something weird happened during the show. In that case, if there were 4 JBL's on the Mac, it would be the full 300 watts being delivered, but still only about 230 watts being delivered to the 3-JBL cab since a portion of that power was diverted to backstage.
Also, it's true about the power rating. The 2300 is fairly clean (.25% distortion) at 300 watts per channel, but peak power jumps quite a bit higher, maybe by 20% to 30% more for brief surges and transients. Somewhere I have some bench test results from a Mc250 and peak power was quite a bit more than the rated 55 or so. But still continuous power ratings are the better number to go by. Most amps will be able to pulse out brief amounts of power but can't sustain it.
I just looked more closely at the actual power graph of the Mc2300, and really it looks pretty clean at levels over 300 watts, more like 350 or so at the same .25% harmonic distortion thru the audio band. So yeah, these things are very conservatively rated.
This discussion on "underpowering" and how that would have the sound people pulling their hair out, well I wouldn't really go there with it. I think you may be reading too much into the idea of underpowering. If you take that theory even further, then a 1 watt amp driving a 3000 watt speaker cabinet would be frying speakers easily. This just isn't the case. For Jerry with his 230 watts or so into the 3 JBL's (450 continuous and 900 watts peak) isn't necessarily a recipe for disaster. In fact, the whole world of guitar amps is ALL about having an amplifier running out of power and overdriving while the speaker doesn't blow. This is the normal world of guitar tone. Even in Jerry's case, the slightly driven McIntosh is where a lot of tone and musicality happens. Again, the idea that JBL was trying to get across is that for cleanly driving PA speakers, you want plenty of power so that IF you drive the speakers at their full rated power that the amplifier power is clean and not distorted.