I have their 991 and it's had no problems four ohm driving Maggie 1.6's...
...but these are a fairly constant four ohms. Your speakers impedance curve may come into play here. i.e. Some speakers drop below four ohms even though they are rated at four ohms (nominal).
Oh, the 991 is rated at 200 wpc @ eight ohms and 300 wpc @ four ohms.