back in the late 70's/early 80's, the hot 'ez-gig' pa was a pair of peavy sp-1 15+horns for mains, a pair of 10+horns for monitors, a tapco 6000 mixer, and a crown dc300 with one channel powering mains, the other monitors. it sounded great, as long as you were only running vocals thru it.
so after a long haitus, i'm back playing live again, but now i have a mackie 408m pushing a pair of 1x12+horn black widows, and a pair of yamaha 1x10+horn for monitors.
the mackie has two 250 watt@2ohm power amps, and you can run either both amps as a mono main, or one side main, one side monitor.
ok, so far so good. it's a great little mixer/amp, in theory....
so, in theory, my 8ohm main speakers should be getting 210 watts, while my 8 ohm monitors are getting around 210 watts too. only vocals are coming thru the speakers.
why on earth am i getting distorted vocals when the mains are at 12 o'clock? the channel volume is at noon too, with the trim high enough to light up the signal light ala' the mackie setup guide. if HALF a 300 watt crown could literally drown out a three piece band running svt's and 50 watt half-stacks, shouldn't 460 watts keep up with small 30 watt combo amps (i've mellowed!)?
the mains speakers i've used with a mackie 1400i on another rig, and i know they can wail, so it's the power amp distorting, not the speakers. i paid way more for the mackie vs. the equivalent yamaha/peavey unit 'cuz i thought it would be a smokin' rig, but perhaps not....
Perhaps I have missed something, but I'm not sure I agree with your math.
Two 8 ohm speakers in parallel gives you a 4 ohm load. You mentioned (correctly) that the Mackie has 250 w/ch at 2 ohms. Since your total load on each side of the Mackie amp is 4 ohms you can assume you'll get "roughly" 125 watts per channel out of it. That 125 watts now has to be dived among two speakers, which means each speaker is actually getting about 62.5 watts.
That's a pretty far cry from what you were getting with a DC-300. If memory serves the DC-300 was rated at 150 w/ch at 8 ohms, so at 4 ohms you were probably getting more like 300 w/ch out of it, which would translate to roughly 150 watts per speaker. And keep in mind that Crown historically was VERY conservative in their amplifier ratings, so you probably got a little more out of it than you thought.
Now, there are other variables such as speaker sensitivity, etc., but I estimate that (based on some simple and crude, but generally close enough calculations) the Crown system was capable of going a good 3 or 4 dB (SPL) louder without distortion. This may not sound like a lot, but it is a noticeable difference. If the speakers you use now are less sensitive or efficient than the old SP-1's the net difference could be significantly greater. You may be thinking, "surely newer speakers would be more sensitive and efficient," but this isn't always the case. Raw efficiency is one thing, but sensitivity is sometimes sacrificed for sound quality. The passive crossover in the speakers you use now does a lot to "contour" the quality of sound. The end result is often a better sounding speaker, but one that requires more power to get to a given volume. If you recall, those SP-1's could take your head right off at times - they weren't exactly the smoothest sounding boxes even by late '70's standards. They were loud, and good enough to get the job done. These days manufacturers generally try to make their speakers much more "pleasant" sounding, and the trade-off is often lower sensitivity.
I speculate that this is at least part of your problem. A good way to get to the bottom of it would be to borrow a good, powerful amp and try it out to see what kind of difference it makes.