I have a XR25 (same amps as XR45) and I tried it out with Dynaudio speakers that measure about 3.7 ohms dc. It sounded fine and didn't get hot. However I can't say how things would go long term. Panasonic must have a reason for the 6-8ohm spec. But perhaps that is related only to maximizing power output...don't know.
One thing you can do is use a subwoofer (powered by its own amp) to take some of the load off the Panny.
One thing you can do is use a subwoofer (powered by its own amp) to take some of the load off the Panny.