Here is where I start to question that general logic.
From what I read all over the net, subs (preferably 2 or more) are pretty well essential for proper HT sound reproduction.
Most speakers, unless stratospherically priced, cannot equal the output & low freqs of a decent powered sub or two.
Furthermore, the sub(s) take the load off the amp/receiver as the LFE is handled by the subs' amps thus allowing only the mids & highs to be driven by the amp/receiver.
For me, bookshelves, each with its own sub, is the most cost effective approach. Others prefer towers & sub(s)...
So, logic would break your statement down into several parts.
I will agree that movies and the general HT require that you reproduce down into the lower octaves where a Sub will do a far better job than the average speaker can. The spec for a THX speaker I have been told is that it can reproduce down to the 80hz. And the THX sub can take over from that cross over point and below. But that is a pretty open point as most decent speakers sold today can do that. Maybe not the small micro bookshelf ones, but most decent sized speaker.
So you look at Axioms M2 and it's pretty good down to 70hz. But a movie needs to get down to what?
So then you look at your comment on stratospherically priced speakers. Well, compared to the M2's the LFR1100 would be considered very expensive at over 10x the cost. And the frequency and loudness of the speaker justifies the cost. It can easily get down to the 32hz mark without too much sound loss.
Then I compare it to my Mirage PS8 subwoofer that is spec'd 29~100hz. So in comparison to the M2's that can work down to 70, the larger M80, M100 or LFR1100, you are only out by a few hz from what a common sub will produce. Even Axiom EP350 only does 26hz.
So going for a larger sub that gets down to the 16hz, or 12hz is far more overkill than any movie will generally produce.
So then you get to your point, you split the power off to another amp channel. Yes, except with the LFR you are already going to a dedicated 4 channel amp to power the two speakers, so you put next to no taxation onto a receiver. And Axioms own ADA1500 is only 300w with 5 channels driven. So my Anthem at 250w isn't too far off. You are right its not 800w like an EP800.
So comes the other question, if you are offloading the deeper base to a sub, then why bother with a tower speaker at all as you are never using the low frequencies.
But then again, a part of me says that if you can get the receiver to still give the sound to your fronts, and supplement the lower freqencies with a sub then you are getting like 2+2 subs in your fronts are acting like a cheap sub and then you have the 2 real ones. I guess it comes down to the ultimate question of how does it sound?