One Monoblock or Two?

First, what do you call a 'difficult load'? Are you just referring to impedance / sensitivity? Or, the rest of the equation. If you have to ask what that is, you need to know.

Please check out the link on bi-amping. good information for down the road.
http://sound.westhost.com/bi-amp.htm

I especially recommend section 1.4

Ghost, instead of saying.....how much power is needed per 'half', I'd have asked what the crossover frequency was.

The 50:50 power point is about 350hz while more power is indeed needed by the lows as crossover goes to higher frequencies.

Also, I think it is just possible that amps have different latencies...That is, the amount of time it takes for a signal to go from inputs to speaker. Even very small differences will result in phase shift near the crosover.

And as a final point, I'd agree that Active crossovers would really open up a can of worms, but if I had deep pockets, that's the way I'd head and start with as close to the 'stock' transfer function as reasonable.

Again, please look at the biamp link I provided. Good stuff:

EPDR takes into account the reactance of the load.
My preference is to go to 'root' and use powerfactor. I also like the power cube way of measuring amp output by taxing the amp with inductive, capactive and resistive loads from 1->8 ohms, and +-60 degrees.....You measure the extremes and the amp is good 'under' the limits measured. The graphic gives a response range for the amp which is intuitive and easy to understand.

Electrostats may just be a wacky hi phase load at higher frequencies. Fortunately, those frequencies in normal music require the least power.

http://www.audiograph.se/Downloads/PowerCube_12p_brochure_complete.pdf

Also, most speakers when measured (why bother? seems to be the attitude) are only measured for impedance, which from the above and EPDR, is only part of the story.
I have been a big fan of a different presentation of the same data in the form of the Smith Chart.
I understand the RF guys like this.(microwave?)
Anyway, it also, when scaled properly, makes perfect sense for speakers, as well.

The 50:50 point for power is AROUND 350 hz, based not on speaker need, which you can't know in advance..or predict for YOUR speakers, but simply on acoustic power. Add more as you see fit, based on how wacky the speaker load is.

What got me thinking about this was a long time ago, the original Braun Tri-amps which had 100 watts RMS per speaker divided 50:35:15 with unknown (to me) crossovers.

Article on the Smith Chart presentation.
http://www.hometheaterhifi.com/component/content/article/228.html

Doing a search for"Equivalent Peak Dissipation Resistance" gave quite a few hits. I was surprised at the number.
In looking thru the first 4 or 5 I came across the stereophile article. In it, the author said something about JA liking the measure, and wanting to use it but not really having the time to implement it.
I suspect it would take some data entry and real number crunching to get it useful. Not to mention the huge database of speakers which already had been measured.
Than, the education part would start. People INSIST that low impedance = 'bad'.
Some speakers, even low sensitivity are tube friendly. I was reading about some guy who had Harbeth LS3/5 copies and some fairly low powered tube amp. I'm guessing it worked like a charm.
People are tough to convince. I'm in that category, too, i'll admit....most of the time.

And, in looking thru another aritlcle, they showed phase / impedance data for several speakers, including an electrostat. OUCH. The trifecta hit. Low impedance, high phase angle AND low sensitivity. How can you top that?

In looking this over, I'm willing to see if I can make sense of this new measure, as soon as I start seeing data.

I'd like to know why speaker designers don't wise up to their amp killing ways? I've been lookiing at measured data for quite a while and while I don't have a 'quality number' to characterize a speaker, big impedance dips at hi phase angle points have always been a red flag...

I urge any reader to look at the Smith Chart and see how this measured data can be put on a single lined chart.....and have it make sense.....