Lee, the reason that the 1550 and so many other receivers that can handle 4 ohm rated speakers with no problem in real-world home use can't claim an "official" 4 ohm rating is the severity of the tests they would have to pass. The FTC(Federal Trade Commission)has regulations for amplifier power ratings which have to be followed for any units sold in the U.S(and which, as a practical matter, are the same as those sold in Canada), and the UL(Underwriters Laboratory)tests for overheating. In brief, for an official 4 ohm rating the unit would have to be operated at the full rated power for at least five continuous minutes into a 4 ohm load. Most units aren't designed for such an ordeal without overheating and having their protective circuits shut them down.
Nevertheless, for actual home use the 1550 and many other receivers are serving their owners well driving speakers, including the M80s, which are rated at 4 ohms. Incidentally, you might also note that the M80s are 8 ohm speakers over the majority of their frequency range.
