I used to believe this, but is it true with modern decent quality AVRs in typical home listening spaces?
Great question but hard to come up with a simple and clear answer. It depends what is "typical home listening spaces". In some countries/cities, it could be about 15X25X(8-10)or 12X18X8, or as small as 10X12X8 ft. So let me try this:
If we go by the THX select standard, then AVRs such as Denon 3000, Marantz 6000, series will be fine for most people, but those with truly 4 ohm nominal impedance, with frequent dips below 3 ohms can still benefit from external amps rated 200 W/300 W 8/4 ohms unless they don't listen loud at all, such as keeping their Denon or Marantz mv at below -20 (before auto setup/Audyssey).
THX Certified Select System which covers spaces up to 2,000 cubic feet in size, with a ~10 foot viewing distance from the screen.
Some, including whoever wrote the FAQ for Magneplan has provided some reasonable guidelines such as the quoted paragraph below:
From Magneplan FAQ:
Everyone understands they need plenty of power, but the role of power supply is not understood. There is one important concept you need to understand when shopping for an amplifier or receiver: and it is somewhat like understanding "good" and "bad" cholesterol. The ratio is very important. An Gold Standard for an amplifier would be to double the power at 4 ohms. This concept is important even if you are buying an 8 ohm speaker. If the amplifier is rated at 80 watts at 8 ohms, it should (ideally) produce 160 watts at 4 ohms (or close to it). None of the receivers will do that. However, this is the benchmark of a good amplifier design. A 10 watt amplifier that produces 20 watts at 4 ohms "speaks volumes" about the PHILOSOPHY of the designer. (But, of course, it does not tell you if a 10 watt amplifier is enough for your room.) In the final analysis, buy an amplifier that comes as close to doubling the power at 4 ohms as your budget will allow.
In my opinion, it is somewhat misleading though, at least the part I underlined. While it is true in a sense, in theory doubling down is near impossible in true term based on electrical principles unless regulated power supply (that will have its own cons) and other techniques are used; and in practical term manufacturers can easily de-rate the output into 8 ohm in order to show 2X the output at 4 ohm. The fact is, as long as the amplifier is truly rated for 4 ohm loads without resorting to switching rail voltage via selector switch, relays, electronic circuitry etc., then who cares if it doubles down or not. For example, will you take a 100 W/200 W 8/4 ohm rated amp over a 150 W/ 200 W 8/4 ohm one, all else being equal? The answer should be obvious.
Some will emphasize it is high current that counts, that kind of statement, while true, is also misleading because then someone could be led to think that a so called high current amp is better for his/her 4 ohm speakers with dips to 2 ohm. When in fact, he/she should just look for an amp that may not be advertised as high current, but has more current capability in absolute sense, than the so called high current one.
Example to demo this point is, take a look of the Harman Kardon AVRs, or receivers in the past that has the impressive 40 to 60 A instantaneous current specs such as the HK AVR630 and compared to a simple try and true Denon AVR-3805 and anyone who understand the basic electrical principles might laugh at HK's "high current" claims. I have the AVR-3805 and I know it was one of the very few AVR of its time that actually pass the 1 ohm load test, none of the HK AVR at that time period did!
I rarely turn the volume above 65 on the Denon AVR.
