Well, in reading the specifications on the Onkyo 595 (which you mentioned was 1 of your short list models), it states "Full bandwidth power to all 5 main channels High-current,low-impedance 4-ohm drive for all five channels ". To me this is definitely an idication they have designed it to drive lower impedances. Most modern amplifiers will drive 4 ohm loads, but some may not do it well.
Remember, impedance is variable over the frequency spectrum. While a speaker may be rated 4 ohm, it does not always represent a 4 ohm load to the amp.
But the point is moot, as you got deal, and it better suited your budget. Remember to purchase better than the thrown in interconnects and speaker cables. Don't go overboard, but you may want to look at other posts. There has been a fair amount of discussion on this topic.