Those shouldn't be considered hard to drive. If you sit 3.2 meters, SPL drops 10 dB, say you have 2 dB room gain so you have a net loss of 8 dB. Let's now assume your drivers in the box resulted in an overall sensitivity of 88 dB/M/2.83V and nominal impedance of 8 ohms, then you are getting 80 dB per watt. Further assume you lose another 3 dB (effectively) due to phase angle issues and whatever other factors, you should still end up with around 77 dB per watt.
Now if you regularly listen to SPL of 70 to 75 dB, the 2311 should still give you about 20 dB of headroom for dynamic swings but if you regularly listen at 80 to 85 dB and occasionally crank beyond that then the 2311 will run out of gas, probably will sound compressed depending on the kind of music, even if not distorted. Whether the resulted sound quality difference will be "drastic" or not could just be a matter of definitions. What you considered drastic I may still consider minor. Surely there could well be many other factors but power requirement is a good place to start especially when you are comparing an entry level 7.1 AVR to a mid range 2 channel power amp that has an over sized power supply in it.
lifestyle: Sorry we digressed, but please make sure you have enough power for your needs.
I'm about 12' from the speakers, my typical maximum listening level with the 2313 was around -20 with no source level boost and no EQ. The volume control on my preamp is usually at about 9:00 with no tone controls being used. I know the sound of a system that's under-powered- the Denon should have had no problem with that and my line voltage doesn't sag- I have checked it many times. I have also used REW, my SPL meter and iPhone app to check the SPL in this room and it was rarely above 90dB. I have used the iPhone app at our Summerfest, which is a big music festival, and I know people who have worked there- they're told to keep the SPL at/below 95dB, but they tend to boost the low end because the meters are weighted. When I'm in that kind of environment, I ALWAYS wear ear plugs and trust me- it's never that loud here.
When I set up AVRs, mostly Denon, I set the maximum output at -10dB because I know that if people can crank it to WOT, they will. Might not be intentional, but it can and does happen. I don't remember the last time a customer called about dead tweeters in a system I installed. I replaced a Monitor Audio tweeter for someone before I did anything else to his system and that went away because he was using the Zone 2 output from his AVR for his living room speakers, which wasn't limited and was operating beyond the capability of the amplifier.
Your calculations are for one speaker- a second speaker receiving equal power adds 6dB and if the reference for sensitivity is measured at 1 meter, it would drop by 6dB at 2 meters
in a non-reverberant field. In a semi-reverberant field it drops less because of, as you mentioned, room gain.
I'm not new to this stuff- I have worked selling, listening and choosing audio equipment for almost 40 years, in residential, commercial and mobile. I have judged car audio contests many times, built many audio systems that not only won or placed well in their power class (sound quality was where the contests were most heavily weighted) and when I see past customers, they mention these systems. One of the compliments I have received many times has to do with getting better sound from low powered systems than other people they knew, who were also in the business. When I did car audio, I would ask the owners for an oscilloscope for making sure the amplifier(s) didn't clip at high levels, but they never saw the value in it. I also did live sound in the past and received compliments on the sound. Well, when the guitar players weren't so loud that I couldn't balance the sound, anyway.
