Yeah, it's a bit of a tricky situation with heat and Onkyo in particular.
On the one hand, there's no denying that many Onkyo receivers produce a great deal of heat. On the other hand, so do many, many other receivers, but they never seem to draw the same flak over it.
The simple fact is that any Class A/B amp - which is the vast majority of receiver amps out there - only turns about half of the electrical current that is drawn into actual Wattage to drive the speakers. The other half all turns into heat! So Onkyo is by no means unique in producing a lot of heat from its amps. The issue seems to be more instances of people reporting heat-related problems, such as shut downs. So it seems to be more about heat dissipation or other components of the receiver being too heat sensitive.
To be fair, I've also seen a number of Onkyo complaints where users really have not followed the instructions. I've seen receivers placed on carpet. I've seen receivers stuffed into cabinets with barely enough space to fit the receiver, let alone allow any air to circulate. I've seen receivers with other components stacked on top of them.
Again, on the one hand, every brand experiences these sorts of user errors and failures to follow the placement instructions. On the other hand, when the instructions ARE followed and the air is allowed to circulate properly around Onkyo's receivers, there tend to be fewer problems - almost as though the instructions were there for a reason
Onkyo does provide a "4 ohms" setting as well. All it does is limit the amount of current to the amps, which naturally lowers the temperature, but also chokes the amount of available power. While you can get more power out of the amp by leaving it in the "6 ohm" or "8 ohm" setting and still drive 4 ohm speakers just fine, you do run the risk of the amps overheating - again, almost as though the 4 ohm setting were there for a reason.
I think what really confuses things for most people is they see the "100 Watts per channel" or whatever the Wattage number is, and they naturally think, "great! So each channel can deliver 100 Watts (or 120 W or 135 W or whatever) at any given moment." Well that isn't really true. Onkyo clearly rates their amps with only 2 channels driven, meaning that the total available power is only twice the per channel rating, not 7x the per channel rating.
So, in truth, there just isn't all that much amplifier power truly available - especially if you are driving 5 or 7 speakers! For a large room, where significant amplifier power is genuinely needed, separate amplification for at least 2 or 3 of the channels is a very good idea! A pair of 200 Watt monoblocks has nearly double the power of the entire "pool" of amplifier power for all 7 channels in the 809 if you're running the 809 in "4 ohm" mode.
So, on the one hand, no denying the high level of heat that Onkyo's receivers can produce. But, on the other hand, if you follow the instructions for placement and air circulation, and limit the amp power by putting it in "4 ohm" mode as Onkyo advises, their receivers don't run any hotter than any other brand, and they rarely have issues. I chalk a lot of the "Onkyo bashing" up to people not following the instructions. However, I also sympathize and understand users' frustration because the specs can be misleading and people are naturally expecting higher output capabilities than what is actually available.
