Hi Blobula,
In the past, a number of Onkyo AV receivers would not drive 4-ohm loads without current-limiting or shutting down. If you examine the specs of most AV receivers except for a few higher-end models (Rotel, B&K. . .), most manufacturers will warn against driving 4-ohm loads.
However, through Axiom customer experience as well as bench-testing a number of different brands at Axiom's plant, we have found that some brands have sufficiently stable amplifier sections to drive the 4-ohm M80s in rooms of reasonable size (not huge rooms) to high volume levels without overheating or current limiting. These brands include Denon, Sherwood Newcastle (models R771 and R865), Harman/Kardon, and Outlaw Audio. I don't know about the Onkyo 604, but many of Sound&Vision magazine's tests of Onkyo AV receivers with 4-ohm loads cause them to go into current limiting. The latter limits the amplifiers' output power to a fraction of the normal output.
Regards
